The gonad histology, gonado-somatic index (GSI), 17 beta-estradiol (E-2) levels and acetylcholinesterase (AChE) activity in the carp species Chalcalburnus tarichi from Lake Van and the Karasu river, eastern Turkey, have been investigated. Fish between 5 and 7 years old were sampled from November 2003 to February 2004. The ratio of female fish caught in Lake Van with abnormal ovaries (AbOF) was 43.3%, but the fork length and body weight of these fish were not correlated with this abnormality. The weight of the ovaries and the GSI values of AbOF were very low (P < 0.05). Histological observations on the samples caught each month revealed that the oocytes had degenerated in the perinucleolus and early cortical alveolus stages and that the ovaries were full of somatic stromal tissue. In addition, the seminiferous tubules of male fish with abnormal testes did not contain male reproductive cells at any stage. The ovaries of the fish caught from the Karasu river were also full of oocytes in the perinucleolus and early cortical alveolus stages, but there were fewer atretic follicles. Furthermore, apoptosis was observed in the ovary cells of these fish, in particular in the follicular cells, and the plasma E-2 levels of the AbOF was very low (P < 0.05). AChE activity was inhibited only in liver (P < 0.05). We conclude that our sample of C. tarichi must have been exposed to various polluting chemicals or another unknown factors (such as global warming) and that these factors have irreversibly impaired oocyte development in a high percentage of fish.