Population levels ofPseudoplusia includens(Walker) were found to vary considerably among agricultural ecosystems in Louisiana. The cotton, corn, and soybean ecosystem was characterized by a dense larval population in late August or September.Parasitism was inversely related to population density and to the amount of pesticides used in the surrounding area.Apanteles scitulusRiley andCopidosoma truncatellum(Dalman) were the predominant parasites, whileLespesia aletiae(Riley) was of lesser importance.The fungal pathogensSpicaria rileyi(Farlow) Charles andMassosporasp. attacked the soybean looper larvae at different periods and densities.S. rileyiappeared in midsummer at relatively low population densities, whileMassosporasp. was present only in late August or September in very dense larval populations.