The virilis group species Drosophila montana is widely distributed around the northern hemisphere. Here we show that it consists of at least two well differentiated populations (Finnish and North-American populations) that have been diverging for the last 0.55-0.95 My. These populations show significant chromosomal, behavioural and morphological differences, but no apparent postzygotic isolation. Evidence for introgression is found for both Finnish and North-American populations at two out of the three X-linked genes (fused, elav and su(s)) studied here. In the light of these findings, previously reported evidence for selective sweeps in D. montana populations is re-evaluated.