The study showed that both larvae and adult males of C. pipiens pipiens and C. pipiens quinquefasciatus from Johannesburg, Transvaal, can be distinguished morphologically. Males were separable either by the structure of their genitalia or according to the proportion of the shaft of the maxillary palp which bears hairs. Ninety six percent of adult females were distinguishable by a quantitative character in the wing venation. Laboratory colonization showed that quinquefasciatus was stenogamous and pipiens eurygamous which suggests that reproductive isolation is due to an ethological premating mechanism. In 9 crossing experiments there was no cross-insemination between the 2 taxa except in one experiment where 2 quinquefasciatus females were inseminated by male pipiens. The F1 + F2 hybrids subsequently reared were vigorous and fertile indicating the absence of postmating isolating mechanisms. After study of these laboratory hybrids a search was made for natural hybrids among mixed field collections from Johannesburg and from Bethulie, Orange Free State. Not one such hybrid was found. From this evidence that pipiens and quinquefasciatus are not interbreeding and because of their considerable morphological difference it is concluded that the 2 taxa in South Africa should be classified as separate species.