<p>Reproductive isolation is an important factor in the origin and maintenance of races and species. Among different types of reproductive isolating mechanisms, sexual isolation (premating or prezygotic) is an important barrier to random mating due to behavioural incompatibilities and constitutes the largest and most important class of reproductive isolation in animals. It has been extensively studied in the genus <i>Drosophila</i>, a very good biological model. When a large number of species were tested for sexual or ethological isolation, this phenomenon was found to be widespread in the genus <i>Drosophila</i>. The species vary in the degree and pattern of sexual isolation, which is often used to discuss their phylogenetic relationship and the direction of evolution between the species. Thus, the study of sexual isolation has evolutionary significance and plays an important role in the process of speciation. Different models have been proposed to discuss the evolutionary sequence of the species. Kaneshiro suggested that the species whose females more often fail to mate with the males of other species is the ancestral species, and the species whose females readily accept the males of other species is the derived one. On the other hand, Watanabe and Kawanishi proposed that it is the females of derived species that do not mate with the males of ancestral species, and the species whose females readily accept the courtship overtures of others is the ancestral species. In the light of these models, the evolutionary sequence between different species of <i>Drosophila</i> is discussed.</p>