AbstractThe Palaearctic speciesDrosophila subobscurahas recently colonized a large area of North America where it coexists withDrosophila pseudoobscura. The viability and developmental rate of these species were studied at 13 d̀C, 18d̀C and 23 d̀C and at densities of 10, 50, 100 and 200 eggs per vial. The two species were differently affected by density and temperature in the ranges studied. Both intra‐ and interspecific cultures showed thatD. pseudoobscurawas best adapted to 23 d̀C, where it was clearly the dominant species. On the other hand, at 18 d̀C and especially at 13 d̀C, althoughD. subobscurawas less viable thanD. pseudoobscura, its developmental time was shorter, which may give advantage to this species. Results reported here agree with the observed distribution of these species in North