Chick embryos were exposed to a teratogenic dose of Malathion at the 5-day incubation stage and the effect of this treatment on the histogenesis of the islets of Langerhans was studied during the 11- to 19-day incubation stages. The Malathion caused a significant increase in the amount of alpha tissue on incubation days 13 and 15, and a significant increase in the amount of beta tissue on incubation days 13, 15, and 17.