This study documents the occurrence of testicular tumors in a wild population of carp–funa hybrids. The most prevalent tumor was a dysgerminoma. There were lower prevalences of seminomas, leiomyomas, Sertoli cell tumors, and spermatocytic seminomas. Sex-ratio, gonadosomic index (G_i) and the prevalence of tumors in the monthly catch was analysed for five consecutive reproduction periods (1980 to 1984). Gonadal tumors were found only in males. This may be important to the population dynamics of the carp–funa hybrid, since a high percentage of the fish that die during spawning have these tumors. A schematic model for the reservoir's population is suggested.