Male and female Egyptian cotton leafworm, Spodoptera littoralis (Boisduval), mating pairs on plants, and males caught in traps were collected at night in pheromonetreated and untreated plots. A polymeric aerosol formulation containing (Z, E)-9,11-tetradecadienyl acetate (5,000 release points per ha; total of 75 g [AI]/ha; release rate of ca. 3 g/ha per day) effectively reduced captures of males in traps and numbers of females copulating in the treated field. Many releasers spaced closely together appear to be needed for effective mating disruption of S. littoralis. Males responded to pheromone traps most actively when the number of virgin females in the population had declined considerably. Male and female Egyptian cotton leafworm, Spodoptera littoralis (Boisduval), mating pairs on plants, and males caught in traps were collected at night in pheromonetreated and untreated plots. A polymeric aerosol formulation containing (Z, E)-9,11-tetradecadienyl acetate (5,000 release points per ha; total of 75 g [AI]/ha; release rate of ca. 3 g/ha per day) effectively reduced captures of males in traps and numbers of females copulating in the treated field. Many releasers spaced closely together appear to be needed for effective mating disruption of S. littoralis. Males responded to pheromone traps most actively when the number of virgin females in the population had declined considerably.