Spontaneous testicular tumors, seminoma, were noticed in four male hybrid catfish (C. batrachus female x C. gariepinus male) after the age of two years. The hybrids showed massive abdominal swelling with catchectic body and free lobulated, encapsulated tumors (> 325 g) within the serosanguinous fluid-filled peritoneal cavities. The tumor cells were large and polyhedral with prominent centrally located nuclei. Other vital organs appeared normal. It seems to be the first report of seminoma in hybrid catfish and possibly of genetic cause.