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Fraud-on-the-market principle is a rule applicable to securities law. According to this rule, the plaintiff in a claim under the antifraud provisions of the federal securities laws may by presumption establish reliance on a misstatement about a security's value without proving actual knowledge of the fraudulent statement if the stock is purchased in an open and developed securities market. The rule is based on the theory that the market price of an issuer's stock reflects all available public information. The rule was established in the case Blackie v. Barrack, 524 F.2d 891 (9th Cir. Cal. 1975).