To test the hypothesis that exposure to organophosphorus (OP) insecticides reduces postfledging survival of altricial birds, 16—d—old European Starlings (Sturnus vulgaris) were weighed and orally dosed with corn oil containing 6.0 mg of dicrotophos per kilogram of body mass or an equivalent exposure of pure corn oil (controls). Two days later, each survivor was weighed again and patagially tagged for identification after fledging. Resightings of marked fledglings were made during weekly 2—d intensive observations yielding four estimates of postfledging survival. Before fledging, only OP—dosed birds died (18.5%), and OP—dosed survivors lost more mass (5.2%, P = .001) than controls (1.4%) but their masses on day 18 were only slightly lower (2% of control mean, P = .10). Brain cholinesterase activity, a sensitive indicator of OP exposure in birds, was depressed and average of 93% in OP—dosed nestlings that died compared to controls, and an average of 46% in OP—dosed nestlings alive on day 18. Age at fledging, postfledging survival, flocking behavior, and habitat use, however, did not differ between OP—dosed and control birds. The effects of the OP on the nestlings appeared to be rapid, to be reversible in survivors, and did not extend into the postfledging period. In addition, we found no relationship between body mass at fledging and postfledging survival.