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Shares trading in the Bolsa Mexicana de Valores do not seem to react to company news. Using a sample of Mexican corporate news announcements from the period July 1994 through June 1997, this paper finds that there is nothing unusual about returns, volatility of returns, volume of trade or bid–ask spreads in the event window. We provide evidence that suggests that unrestricted insider trading causes prices to fully incorporate the information before its public release. The paper thus points toward a methodology for ranking emerging stock markets in terms of their market integrity, an approach that can be used with the limited data available in such markets.