Data are reported showing large directional changes in the frequencies of some gene arrangements and arrangement combinations in certain natural populations of Drosophila robusta in the eastern United States. The changes involve the same X-chromosomal inversion differences in two of the three localities studied and similar autosomal inversions in all three. These genetic changes provide a rare opportunity to observe evolutionary forces at work in nature. They are interpreted as being due to natural selection.

Corresponding Editor: M. Noor