Seasonal population densities ofHeliothis virescens(F.) andH. zea(Boddie) in the Imperial Valley of California were determined by pheromone-baited trap catches and by egg and larval densities in alfalfa, cotton, and lettuce. Moth populations were present from March to December; low densities during winter and early summer corresponded to conditions favoring winter diapause and summer estivation, respectively. The relative importance of each host in supporting moth populations varied seasonally. The dominant species in alfalfa wasH. zea, whereasH. virescenswas dominant in cotton and lettuce. The highest population ofH. zeain alfalfa occurred in late spring and late summer, in cotton during early summer, and in lettuce during October. The highest populations ofH. virescensin alfalfa were found during October, in cotton from July through October, and in lettuce from October through December. Populations in alfalfa and cotton were greater in commercial fields than pesticide-free untreated fields, whereas lettuce populations were greater in untreated fields. Larval mortality from parasitoids and viruses was infrequent except in alfalfa during late spring and early summer.