Economic infestations of both bollworm (Helicoverpa zea) and fall armyworm (Spodoptera frugiperda) developed in South Carolina during late July 1996 in each of 3 large (5A) plots of B.t. cotton (NuCotn 33b) that were treated with high rates of acephate (0.75 lbs active) on June 24 and July 2 to disrupt beneficials. Adjacent plots of similar size that were not treated exhibited no economic infestations of either pest in any of the 3 on-farm fields. Populations of geocorids and ants were decimated in acephate-treated plots and did not recover to effective levels by mid-August. Other beneficials exhibited varying degrees of mortality. After the second application, spiders were reduced to one-third of levels in untreated plots, but tended to recover better than most predacious arthropods. Acephate treatments in each field caused substantial aphid "flare-ups", which were accompanied by similar increases in coccinellid populations in treated plots. In separate observations, exceptionally high populations of large bollworms (1/plant) and fall armyworms (1/meter of row) were recorded in early August in several fields of NuCotn 33b that were not treated for bollworm and were in an area sprayed 4 or more times in early-season with malathion under the Boll Weevil Eradication Program. Our data confirm that applications of broad spectrum insecticides in early to mid-season can compromise the effectiveness of B.t. cotton by disrupting populations of beneficials. Such applications should be avoided if possible and B.t. cotton should be managed carefully for insect pests.