The Old World species, Drosophila subobscura, has recently invaded North America and become sympatric with the native obscura-group species. This study investigates the summer diurnal activity patterns of two northwestern North American populations of Drosophila subobscura and compares them to the documented Old World trend as well as that of its native North American congener, D. pseudoobscura. Both species peak in activity at temperatures between 15 and 20 C, and activity in both species increases with decreasing sun angles, thus causing high levels of activity near sunrise and sunset. However, D. subobscura becomes more abundant than D. pseudoobscura at lower temperatures and sun angles. No differences in diurnal activity were noted between Old World and New World D. subobscura. Drosophila subobscura might have contributed to the reduced abundance of a native species, D. persimilis, in areas of recent sympatry, though this causal connection is not certain.