The seed beetle, Callosobruchus maculatus (F.), exhibits a crowding-induced dispersal polymorphism; high densities of larvae trigger development of dispersing or “active” beetles. This study demonstrates that a genetic component also influences morph determination. Reciprocal, single-pair crosses were performed between a monomorphic, laboratory population and a polymorphic, wild population, and all larvae were reared in a common, crowded environment. Morph ratios in the hybrids were intermediate to those of the parental strains, but displayed a strong maternal effect. The dispersal polymorphism of C. maculatus appears to be controlled by a complex genotype/environment interaction, and contrasts with the purely genetic wing dimorphisms in other Coleoptera.