Using the 'event study' method, we measure the impact on agricultural biotechnology firm equity values of new regulations and other limitations placed on the marketing of biotech crops. Unanticipated declines to biotech firm stock prices indicate that newly imposed regulatory restrictions appear most likely to diminish profit expectations for these firms. The stock price reaction to decisions by crop handlers, processors, or retailers to segregate or limit the use of biotech crops and to reports questioning their safety are less pronounced. Incentives to engage in new biotech crop research and development may be diminished by these developments.