A method for mass production of triploid carp was developed by the cold treatment of eggs after fertilization. Triploidy was demonstrated by cytophotometry and chromosome preparation techniques. Of the haploid embryos, 100% showed morphological abnormalities and died soon after hatching. Haploid embryos of maternal and paternal origin occurring in the course of cold treatment were distinguished by using a genetic marker. Among morphologically normal larvae, the frequency of triploidy was 100%, and the viability of the triploid embryos, larvae and the juvenile fish did not differ from the diploid control. Sex differentiation started in the triploid fish, but was greatly retarded. Gonad size and the small number of growing oocytes suggest that most of the triploid individuals will be sterile. Triploid carp grew at the same rate as diploid controls in the laboratory test.