The use of a semi-natural medium (Eagle's MEM supplemented with chicken embryo extract) appears to be satisfactory for ovarian and testicular fragment cultures of Carassius auratus. The male germ cells show a normal degeneration which might be attributed to a lack of pituitary hormones: gonadotropic and, possibly, somatotropic hormone. The ovarian fragments maintain a normal organization during 21 days of culture. Only the oocytes in late vitellogenesis massively atresiate at the beginning of culture. The addition of fetal bovine serum to this medium produces surprising effects on the testicular explants, particularly on the spermatogonial mitoses. This might be attributed to the presence of STH and androgen in the serum. The modifications of ovarian explants, on the other hand, are identical to those observed with Eagle's MEM supplemented with chicken embryo extract. M 199 has been tested as a synthetic medium; the preliminary results are encouraging. Testosterone, when added to the semi-natural medium, permits complete spermatogenesis in the testicular fragments during the 21 days of culture, but does not produce any effect on the ovarian explants.