A family of serially transplanted melanomas in gerbils is described. These tumors were derived from the cutaneous melanotic melanoma that arose in 1 of 44 gerbils injected postnatally with N-ethyl-N-nitrosourea. It consists of a slow growing heavily melanotic parental line and two fast growing melanotic (FGM) and amelanotic (A-FGM) lines that appeared abruptly during serial transplantation of the parental tumor. The FGM melanotic line originated after a sudden acceleration of growth of the parental line during the 4th in vivo passage that was accompanied by a decrease in both pigmentation and metastasizing potential. The A-FGM derived from the depigmented tissue of the 7th in vivo passage of the FGM line and has been characterized by an amelanotic phenotype, an increased metastasizing potential and similar growth rate to that of the FGM. Once established, both lines expressed considerable phenotypic stability during serial transplantation in gerbils. Thus, the Zeman UJ melanomas represent the first established family of transplantable melanomas in gerbils, which serve as a model for pigmented cell and melanoma research and as a subject for a retrospective analysis of the phenomenon of tumor evolution.