The evolutionary effects of crowding on male courtship behavior were studied using wild and mass-reared medflies. Mass-reared strains had been raised under highly crowded conditions in mass-rearing facilities for approximately 75, 180, and 238 generations. Pre-mounting courtship was facultatively shortened in both wild and mass-reared males under conditions of greater crowding. The courtship behavior of males of mass-reared strains was also shorter than that of wild males under similar conditions of crowding. Shorter courtships are probably advantageous for males in crowded conditions because they reduce the likelihood of the courtship being interrupted by other flies. Several types of data indicated that males rather than females were responsible for shortened courtships. We conclude that heritable variation in male courtship behavior has persisted in a wild population despite its overall relatively low genetic variability, and that genetic changes in mass-reared strains have altered the range of facultative adjustments in courtship behavior.