Drosophila subobscura and D. azteca are closely related species that have coexisted on the west coast of North America since that area was colonized by D. subobscura in the late 1970s. We have studied competition between the two species by the serial transfer technique at different initial proportions (20%, 50%, 80% and 100%) and at two densities (20 and 100 individuals). The cultures were maintained at 18 C. In the mixed cultures D. subobscura outcompeted D. azteca at both densities and all initial proportions. Survivorship was similar for both species, although the productivity of D. azteca was reduced, especially in the mixed cultures. The productivity of D. subobscura in the mixed cultures was a function only of the initial number of individuals of this species, and was completely unaffected by the number of D. azteca in the populations. Carrying capacity was attained by both species when the number of founder individuals was twenty; however, the number of descendants was much lower for D. azteca than for D. subobscura. In the pure cultures an important effect of season was detected in the replicates: the productivity of D. subobscura was much higher in spring than in autumn, while the reverse was observed in D. azteca. This effect was not detected in the mixed populations. Thus, lower carrying capacity, lower fecundity, longer mean eclosion rate, delayed oviposition or a combination of all of them could explain the low success of D. azteca in the mixed cultures.