The threespine stickleback (Gasterosteus aculeatus L.) is a highly varied and widely distributed species of small fish. Throughout most of its range there are two distinct forms. One form (trachurus) is anadromous and the other form (leiurus) is sedentary and lives entirely in fresh water. Hagen (1967) concluded that these two forms were re productively isolated and that both leiurus and trachurus should be considered as species. Hagen's conclusion was based on differences he found in ecological habitats, breeding times, and the apparent absence of introgression between, leiurus and trachurus. The purpose of this study was to determine if ethological factors or mate selection contribute to reproductive isolation. Mate selection tests were conducted in which (1) courting males were simultaneously exposed to receptive females of both types, and (2) receptive females were simultaneously exposed to both types of courting males. The results of both these tests clearly demonstrate that matings between similar phenotypes occur significantly more often than matings between dissimilar phenotypes. Additional experiments attempt to document the specific behavioural mechanisms responsible for mate selectivity. The orientation of males to females, and females to males, was observed and quantified. Males were found to orient, or direct most of their courtship, to females of their own phenotype. Females also tended to orient to males of their own phenotype. Certain behavioural patterns of male courtship were quantified and compared between leiurus and trachurus. Significant differences were found in a number of courtship activities: leiurus males perform significantly more zig-zags per minute than trachurus males, the first courtship response of leiurus males is usually a zig-zag while the first response of trachurus males is usually a bite. Non-random mating between leiurus and trachurus may contribute greatly to their reproductive isolation. Non-random or selective mating may be explained by difference in behaviour between the two forms, particularly during courtship. / Science, Faculty of / Zoology, Department of / Graduate