Avidin is a glycoprotein found in chicken egg white, that sequesters the vitamin biotin. Here we show that when present in maize at levels of > or =100 p.p.m., avidin is toxic to and prevents development of insects that damage grains during storage. Insect toxicity is caused by a biotin deficiency, as shown by prevention of toxicity with biotin supplementation. The avidin maize is not, however, toxic to mice when administered as the sole component of their diet for 21 days. These dates suggest that avidin expression in food or feed grain crops can be used as a biopesticide against a spectrum of stored-produce insect pests.