Gonadal development during the first reproductive cycle is studied in rainbow trout Parasalmo mykiss cultivated at approximately 20°С. The cultivation of the fish at this temperature leads to an increasing proportion of lipid and stromal tissue in ovaries, decrease of the number of previtellogenous oocytes, and to subsequent retardation of sexual maturation in the majority of females. At the age of 2 years, only 36.6% of the fishes reach sexual maturity. Shortly before sexual maturity, total resorption of oocytes of the older generation close to definitive condition is observed. In the males, the period of the beginning of active spermatogenesis wave is substantially protracted. Total resorption of mature spermatozoa is registered in all males just before sexual maturity.