Seasonal dynamics of the adult copepods number and the sex ratio in a natural Epischura baicalensis population of South Baikal at depths 0–50 m and 50–250 m during 2001–2004 was investigated. Most seasonal fluctuations of the adults number and the sex ratio in upper layer were observed. In spring high adults number and sex ratio clearly to 1:1 at upper layer but in summer – at depths 50–250 m were observed. In 2004 the peak phase at depths 50–250 the male domination above females was noted that due to action density-dependent factors. The possible variants of density-dependent mechanisms to control number in a natural Epischura baicalensis population of South Baikal were discussed.