Abnormal sexual development in fish has become the topic of interest in scientific circles. Many studies have shown that intersexual changes are present in population of wild fish and also in marine fish. An analysis of plasma that was collected from male adults revealed high concentrations of vitellogenin and also the presence of oocytes in testicular tissue. Those changes are sign of feminization of the reproductive system. This phenomenon was found in fish from the area of water pollution that was caused by endocrine disrupting chemicals (EDCs). Endocrine disruptors are both natural and synthetic compounds that are present in various products such as plastic, pesticides, fungicides, medicines and cosmetics. Influence of EDCs is based on its agonistic and antagonistic effects on the endocrine system. The most common method that was used to determine fish feminization is hystological examination of testicles from male adults. Biomonitoring is carried out with the aim of early detection of the influence of EDCs. The method is based on biomarkers and is very important in the assessment of environmental change.