Nests of the neotropical bees Exomolopsis globosa differ from those of E. similis in the presence of a second entrance, in the angle of the longitudinal axes of the cells and in the type of ground underlying them. In both species two or three females begin to build a nest and most emerging females remain associated with their natal nests. Adult females co-operate in nesting and probably not all of them lay eggs. Developmental mortality was greater in the larger nests of E. globosa than in the smaller ones. It is suggested that several adult females inhabit each nest because this behaviour reduces the risks the bees would encounter if they searched for new nest sites.