Reinforcement is the process by which natural selection strengthens sexual isolation between incipient species, reducing the frequency of maladaptive hybridization and hence completing reproductive isolation. Although this model of speciation was once widely accepted, its plausibility and experimental support have been recently attacked. Here we provide an example of speciation by reinforcement, in the North American fruitfly Drosophila pseudoobscura. The results suggest that females of D. pseudoobscura evolved increased sexual isolation from their sibling species, D. persimilis, by natural selection against maladaptive hybridization.