Carp sperm with genome inactivated by gamma irradiation ( 6 0 Co; 1400 Gy dose) was used to induce gynogenetic development in tench (Tinca tinca). The non-participation of paternal genome in tench female carp male hybrid diploidized with cold shock was confirmed morphologically, karyologically and biochemically. Consistent yields of 5-21% viable gynogenetic fry were obtained when eggs were incubated at 20°C and cold-shocked (30 min at 0 to +2°C) 2, 5 (P < 0.1) and 10 min (P < 0.05) after gamete activation. When expressed in terms of τ 0 (duration of one mitotic cycle of synchronous cell division related to water temperature) times after activation were 0.069, 0.173 and 0.346 τ 0 , respectively. In a preliminary study of sex reversal, sex was reversed with tench. The greatest effect of treatment appears to lie in the range of a 0.102 g/kg dose of 17α-methyltestosterone (MT) starting at 40-60 days after activation.