Abstract: Genetic markers (sex-specific loci of Ga1 and Ga2) have been used to analyze the sex ratio of the threespine stickleback at different stages of ontogenesis. The progeny of all individually tested females includes both males and females, and no significant departures from a 1 : 1 sex ratio has been found in embryos from the natural nests. Juveniles did not demonstrate a sex bias up to one-and-a-half or two months of age, but in older juveniles, females prevailed (this fact has been reported for the first time). Females prevail among spawners, which mostly results from the predominant elimination of males during the spawning and post-spawning periods. The possible causes of this predominance are discussed.