Elevated temperature causes degeneration and disappearance of the germ cells in the males of scrotal mammals. It was recently shown that heat-induced germ cell degeneration occurs also in fish but, unlike in mammals, it occurs not only in males but also in females. The purpose of this study was to clarify the histological process and dynamics of heat-induced germ cell disappearance in pejerrey Odontesthes bonariensis larvae and juveniles. Monosex and mixed-sex fish produced by thermal manipulation of sex (temperature-dependent sex determination) were subjected to 29°C for periods between 1 and 12 weeks, and used to analyze, by histological methods, the changes in gonadal size and the number of normal and degenerating germ cells. Groups exposed to 29°C for 8–12 weeks were subsequently transferred to 24°C to verify if any gonadal damage would be permanent. Germ cell degeneration, histo-logically characterized by nuclear pyknosis or eosinophilia and cytoplasmic eosinophilia, was observed with increasing frequency at higher temperatures (29>24> 17°C) and more in males than in females. Clear degenerative changes in the germinal epithelium usually began within one week of exposure to 29°C and appeared clearer in females than in males. Complete loss of germ cells was observed only in individuals exposed for periods of 8–12 weeks to 29°C but no treatment produced 100% sterile fish. Germ cells that remained in the gonads after exposure to 29°C retained the capacity to rapidly recolonize germ cell-depleted areas, suggesting that the associated somatic cells in the gonads are little or not affected by this temperature. J. Exp. Zool. 297A:169–179, 2003. © 2003 Wiley-Liss, Inc.