EN
The aim of this research was to explain the etiology of pathological changes that have occurred on the body of goldfish (Carassius auratus). All samples were obtained from a fish farm that utilizes cooling water from the power station Dolna Odra. Since 2008 a few such cases have been sporadically observed there. An excised part of the tissue was analyzed based on histological assays and also by means of a transmission electron microscope (TEM). Histological staining showed that the tumor originated from the mesenchymal tissue with many blood vessels in its structure. The conducted research showed neither pathological changes caused by oncogenic viruses nor viral particles in the tumor tissue. Nevertheless, the most probable reason for these kind of changes is a combination of environmental factors and In Poland specific breeding conditions (warmed waters).