The oviposition biology of the bertha armyworm, Mamestra configurata Walker, was studied with emphasis on the effect of conspecific eggs on oviposition site selection. Bertha armyworm lay clusters of up to 700 eggs, and larvae have feeding and growth habits similar to those of other Lepidoptera that gain advantages from feeding aggregations. In a field-cage experiment, multiple egg masses per leaf were noted, although the vast majority (85% ) of leaves available for oviposition received no eggs. A series of dual-choice laboratory experiments was conducted using paired excised leaves with and without eggs or egg-wash extracts. Females strongly preferred to oviposit on leaves with eggs of a different female than on leaves without eggs. However, females did not prefer leaves with their own eggs over control leaves without eggs. Gravid females also preferred leaves treated with a methanol egg-wash over leaves treated only with methanol, indicating that the source of oviposition stimulation may be chemically based. The potential relevance of these observations is discussed in the context of host-plant distribution and their exploitation by bertha armyworm.