According to reinforcement theory, sexual isolation between species in sympatry is strengthened by natural selection against maladaptive hybrids. Drosophila pseudoobscura and D. persimilis from four locations where these species are sympatric, and from three locations where only D. pseudoobscura has been found, were utilized in studies of sexual isolation. Multiple-choice observation chambers were used to record matings between sympatric and allopatric strains of the two species. There was a wide variation in sexual isolation between the two species in the four localities we studied. The average isolation index for sympatric strains of the species was not significantly different from the average index for allopatric strains. There were no meaningful differences between the isolation indices in sympatric and allopatric strains of the species. The failure to find a relationship is likely the result of gene flow among populations within the two species.