The Foreign Trade Antitrust Improvements Act of 1982 (FTAIA) excludes from the Sherman Act’s reach much anticompetitive conduct that causes only foreign injury. It does so by setting forth a general rule stating that the Sherman Act ‘‘shall not apply to conduct involving trade or commerce … with foreign nations.’’ [Citation.] It then creates exceptions to the general rule, applicable where (roughly speaking) that conduct significantly harms imports, domestic commerce, or American exporters.

We here focus upon anticompetitive price-fixing activity that is in significant part foreign, that causes some domestic antitrust injury, and that independently causes separate foreign injury. We ask two questions about the price-fixing conduct and the foreign injury that it causes. First, does that conduct fall within the FTAIA’s general rule excluding the Sherman Act’s application? That is to say, does the price-fixing activity constitute ‘‘conduct involving trade or commerce … with foreign nations’’? We conclude that it does.

Second, we ask whether the conduct nonetheless falls within a domestic-injury exception to the general rule, an exception that applies (and makes the Sherman Act nonetheless applicable) where the conduct (1) has a ‘‘direct, substantial, and reasonably foreseeable effect’’ on domestic commerce, and (2) ‘‘such effect gives rise to a [Sherman Act] claim.’’ [Citation.] We conclude that the exception does not apply where the plaintiff’s claim rests solely on the independent foreign harm.

To clarify: The issue before us concerns (1) significant foreign anticompetitive conduct with (2) an adverse domestic effect and (3) an independent foreign effect giving rise to the claim. In more concrete terms, this case involves vitamin sellers around the world that agreed to fix prices, leading to higher vitamin prices in the United States and independently leading to higher vitamin prices in other countries such as Ecuador. We conclude that, in this scenario, a purchaser in the United States could bring a Sherman Act claim under the FTAIA based on domestic injury, but a purchaser in Ecuador could not bring a Sherman Act claim based on foreign harm.

The plaintiffs in this case originally filed a class-action suit on behalf of foreign and domestic purchasers of vitamins * * * alleg[ing] that petitioners, foreign and domestic vitamin manufacturers and distributors, had engaged in a price-fixing conspiracy, raising the price of vitamin products to customers in the United States and to customers in foreign countries.

* * * The District Court dismissed their claims. [Citation.] * * *

A divided panel of the Court of Appeals reversed. * * *

The court assumed that the foreign effect, i.e., higher prices in Ukraine, Panama, Australia, and Ecuador, was independent of the domestic effect, i.e., higher domestic prices. [Citation.] But it concluded that, in light of the FTAIA’s text, legislative history, and the policy goal of deterring harmful price-fixing activity, this lack of connection does not matter. [Citation.] * * *

We granted certiorari to resolve a split among the Courts of Appeals about the exception’s application. * * *

The FTAIA seeks to make clear to American exporters (and to firms doing business abroad) that the Sherman Act does not prevent them from entering into business arrangements (say, joint-selling arrangements), however anticompetitive, as long as those arrangements adversely affect only foreign markets. [Citation.] It does so by removing from the Sherman Act’s reach, (1) export activities and (2) other commercial activities taking place abroad, unless those activities adversely affect domestic commerce, imports to the United States, or exporting activities of one engaged in such activities within the United States.

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[The FTAIA] initially lays down a general rule placing all (non-import) activity involving foreign commerce outside the Sherman Act’s reach. It then brings such conduct back within the Sherman Act’s reach provided that the conduct both (1) sufficiently affects American commerce, i.e., it has a ‘‘direct, substantial, and reasonably foreseeable effect’’ on American domestic, import, or (certain) export commerce, and (2) has an effect of a kind that antitrust law considers harmful, i.e., the ‘‘effect’’ must ‘‘giv[e] rise to a [Sherman Act] claim.’’ [Citation.]

We ask here how this language applies to price-fixing activity that is in significant part foreign, that has the requisite domestic effect, and that also has independent foreign effects giving rise to the plaintiff’s claim.

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* * * [W]e base our decision upon the following: The price-fixing conduct significantly and adversely affects both customers outside the United States and customers within the United States, but the adverse foreign effect is independent of any adverse domestic effect. In these circumstances, we find that the FTAIA exception does not apply (and thus the Sherman Act does not apply) for two main reasons.

First, this Court ordinarily construes ambiguous statutes to avoid unreasonable interference with the sovereign authority of other nations. [Citations.] This rule of construction reflects principles of customary international law—law that (we must assume) Congress ordinarily seeks to follow. [Citations.]

This rule of statutory construction cautions courts to assume that legislators take account of the legitimate sovereign interests of other nations when they write American laws. It thereby helps the potentially conflicting laws of different nations work together in harmony—a harmony particularly needed in today’s highly interdependent commercial world.

No one denies that America’s antitrust laws, when applied to foreign conduct, can interfere with a foreign nation’s ability independently to regulate its own commercial affairs. But our courts have long held that application of our antitrust laws to foreign anticompetitive conduct is nonetheless reasonable, and hence consistent with principles of prescriptive comity, insofar as they reflect a legislative effort to redress domestic antitrust injury that foreign anticompetitive conduct has caused. [Citations.]

But why is it reasonable to apply those laws to foreign conduct insofar as that conduct causes independent foreign harm and that foreign harm alone gives rise to the plaintiff’s claim? Like the former case, application of those laws creates a serious risk of interference with a foreign nation’s ability independently to regulate its own commercial affairs. But, unlike the former case, the justification for that interference seems insubstantial. [Citation.] Why should American law supplant, for example, Canada’s or Great Britain’s or Japan’s own determination about how best to protect Canadian or British or Japanese customers from anticompetitive conduct engaged in significant part by Canadian or British or Japanese or other foreign companies?

We recognize that principles of comity provide Congress greater leeway when it seeks to control through legislation the actions of American companies, [citation]; and some of the anticompetitive price-fixing conduct alleged here took place in America. But the higher foreign prices of which the foreign plaintiffs here complain are not the consequence of any domestic anticompetitive conduct that Congress sought to forbid, for Congress did not seek to forbid any such conduct insofar as it is here relevant, i.e., insofar as it is intertwined with foreign conduct that causes independent foreign harm. Rather Congress sought to release domestic (and foreign) anticompetitive conduct from Sherman Act constraints when that conduct causes foreign harm. Congress, of course, did make an exception where that conduct also causes domestic harm. [Citation.] But any independent domestic harm the foreign conduct causes here has, by definition, little or nothing to do with the matter.

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Second, the FTAIA’s language and history suggest that Congress designed the FTAIA to clarify, perhaps to limit, but not to expand in any significant way, the Sherman Act’s scope as applied to foreign commerce. [Citation.] And we have found no significant indication that at the time Congress wrote this statute courts would have thought the Sherman Act applicable in these circumstances.

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Taken together, these two sets of considerations, the one derived from comity and the other reflecting history, convince us that Congress would not have intended the FTAIA’s exception to bring independently caused foreign injury within the Sherman Act’s reach.

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For these reasons, the judgment of the Court of Appeals is vacated, and the case is remanded for further proceedings consistent with this opinion.