To improve fisheries yield and control algal blooms silver carps (Hypophthalmichthys. molitrix) were introduced to Lake Balaton in 1972. Silver carps were introduced to Lake Balaton between 1972−1983. During this period, 1.5 million pieces of silver carp with 3000 tons of total biomass, with an average weight of 200-250 grams were stocked. The phenotypic identification showed that the 90 % of the studied fish were hybrids. Our aim was to explore the seasonal changes in the fecundity and gonado-somatic index (GSI %) of silver carp in Lake Balaton. Between March and November of 2011, 35 female silver carp were collected. Body mass of fish ranged between 13−35 kg (20.5±5.1; mean±SD). The GSI varied between 12-28% (19±4%), while the total number of eggs in gonads ranged between 0.82×105 and 3.71×105 (1.61×105±0.84×105). Compared to other stocks, both GSI and fecundity of silver carps are high in Lake Balaton. In addition, seasonal changes in the number and development state of eggs indicated that silver carp may spawn in Lake Balaton