The lethal effects of heptachlor on bobwhites (Colinus virginianus) were studied at the Patuxent Wildlife Research Center during 1962 and 1963. A pair of adult birds was placed in each of 32 wire-covered field pens (20 x 50 x 61/2 ft) in May, 1962, and in 36 additional pens in May, 1963. Heptachlor (technical grade on clay granules) was spread in the pens at rates of1/8, 1/4, 1 1/4 and 2 lb/acre. In 1963 the treated pens were thoroughly soaked with a hose before birds were introduced to avoid the possibility of birds pecking granules. Heptachlor at 1 1/4 and 2 lb/acre caused severe mortality among birds introduced successively during the first 15 days after application. Mortality declined rapidly among birds placed in these same pens thereafter and was undetectable after 45 days. Mortality at 1/4 lb/acre was significant only during the first 15 days of the 1962 test. There was no detectable difference in mortality of the sexes. Water applied to the pens followed by fairly heavy rain did not significantly affect mortality.