Incipient sexual isolation was found between the subspecies Drosophila pseudoobscura bogotana (Ayala and Dobzhansky) and D. p, pseudoobscura (Frolova). Males took longer to initiate courtship with heterosubspecifics than consubspecifics. This divergence could represent an early step in the speciation of these taxa. The subspecies' pheromones do not appear to have diverged, so the difference is attributed to female behavior.