Pre-larvae hatching fecundity and ovarian fecundity of narrow-clawed crayfish populations were estimated in the lakes and reservoirs of Belarus. Statistically significant differences in the fecundity of females from various water bodies were noted only for Lake Kichino. For all other water bodies, the fecundity dependence on the length of females did not differ statistically and was described by one equation. The greatest loss of eggs was noted during the spawning period (oviposition), insemination and attachment to pleopods of the female. During this short period (2–3 hours), egg mortality of females 120 mm TL is 30 %, it decreases for small females and increases for large ones. During the gestation period, egg mortality averages 8 %.