The ovarian structure and development in juvenile perch, Perca fluviatilis (L.) bred from the moment of hatching under 24-hour illumination and water temperature of 23°C were studied. In 4-10-month-old perch, the size of the lamellae with oocytes grew towards the opposite ovarian walls and often touched them. Oocytes in lamellae were arranged in rows along the lamellae and according to age from the oldest to the youngest towards the germinal epithelium. The oocytes could be divided into three groups in respect to colour, diameter and number of nucleolus: early oocytes, younger generation previtellogenic oocytes and older generation previtellogenic oocytes. The symptoms of cytological sexual maturation manifested by oocyte vacuolisation were observed in several 10-month females. The vacuolised oocytes atrophied. Based on the results of the study it was found that in perch bred under 24-hour illumination and stable water temperature of 23°C, the developed previtellogenic oocytes undergo vacuolisation followed by atrophy.

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