The Catarina scallop Argopecten ventricosus is a true functional hermaphrodite, in which male and female gamete cells develop synchronously in one gonad organ spatially divided into a testis and an ovary. Tetraploid Catarina scallops were produced and macroscopic observations showed a significant reduction in the total area allocated to the testis of this functional hermaphrodite when compared with diploids. Further micro and ultrastructure analyses indicated that gamete cells are larger in tetraploids than diploids and that the clear separation between sexual gonadal areas usually observed in diploids was partially lost in tetraploids, with some male acini projecting into the ovary area adjacent to the testis area. The reduced testis area was present in two generations of tetraploids mated between them, indicating its inheritance. A genetic mechanism of sex determination similar to that observed in the nematode Caenorhabditis elegans is proposed to explain the reduced testis area in tetraploid scallops and the rare occurrence of “only males” or “only females” in this and other true hermaphroditic scallops.