A 2.2 ha-cedar and mixed deciduous tree woodlot in Crawford, Mississippi, harboring 330,000 blackbirds (Icterinae) and European starlings (Sturnus vulgaris) was aerially sprayed by helicopter the night of 1 March 1989 with 1050 1 of a CPT-avicide formulation at a rate of 49 kg CPT/ha. Most mortality occurred within 36 h of treatment. Mortality in the roost was 3% of the pretreatment population. No reliable technique to estimate out-of-roost mortality was identified. Pilot misapplication and probable CPT volatilization of the spray formulation contributed to the low mortality. Only 1 of 37 radiotagged blackbirds using the roost the night of the spray was verified as a CPT-related mortality. Preand posttreatment numbers of hawks and owls in and near the roost did not differ (P > 0.05). Dead nontarget animals found in and near the roost included 10 cardinals (Richmondena cardinalis), 1 robin (Turdus migratorius), and 1 least shrew (Cryptotis parva). Additional research is needed to develop methodologies for evaluating the efficacy of slow-acting toxicants. Proc. 14th Vertebr. Pest Conf. (L.R. Davis and R.E. Marsh, Eds.) Published at Univ. of Calif., Davis. 1990.