Sexual selection can be defined as a mode of natural selection which acts on an individual’s ability to successfully court and copulate with a mate. In most cases, it is the female that selects for specific traits that males possess as males compete for females. When female preferences diverge between species, this can lead to sexual isolation between species reducing or preventing the exchange of gene flow. While the role of female preferences is well known, it is still unclear whether males also show sexual discrimination when it comes to courting foreign females. Because males initiate courtship, they could be the first "line of defense" in sexual isolation. To address this question, we studied sexual isolation among five closely related taxa of the Drosophila affinis subgroup, including three 'races' of Drosophila athabasca: West Northern, Eastern A, and Eastern B, and two more distant species, affinis, and algonquin. We first conducted no-choice mating trials, by pairing males and females of the same or different species and assayed to what extent males court their own versus foreign females. In most cases, we found that heterospecific pairs had lower rates of courtship compared to conspecific pairs. This result was strongly correlated with geographical overlap (allopatry versus sympatry) between species pairs: sympatric species pairs exhibited much higher levels of male courtship discrimination in comparison to allopatric species pairs. This supports reinforcement in male courtship behavior, suggesting that males have evolved courtship discrimination mostly against females that reside in the same geographical area. Furthermore, we hypothesized that altering female pheromones could “trick” a male into courting a female of a different species. To do this, we conducted pheromone "rub-off" experiments and then assayed for male courtship discrimination. We found that males failed to court altered pheromone females, whether the females were of their own or foreign species. This suggests that males target pheromones to assess female species identity. Overall these results demonstrate that males play a major role in sexual isolation between species of Drosophila affinis subgroup and strengthen their degree of courtship discrimination against those species that reside in the same geographical area. This is perhaps the first evidence of reinforcement in male courtship discrimination in nature.