Abstract: Heat shock was used to induce allotetraploidy in Japanese phytophagous crucian carp (JPCC) eggs fertilized anificially with sperms from red crucian carp (RCC) 37-38min after fertilization and shortly before cleavage. Milt was obtained from 1- and 2-year old h4n JPCC male, but ovulated females were not found, and back crossed to the 2n JPCC female produced a large number of offsprings. Chromosome analysis of the offsprings, based on cultured leucocytes from peripheral blood indicated that all were triploids (3n = 150). The fertilizing ability of the offspring was very ;ow (11.4-51.3% of fertility, average : 32.4%). The present observahon supports the hypothesis that the head of the diploid spermatozoas being wider than the haploid may prevent their penetration through the ndcropyle canal. Females or males of interploid triploid JPCC did not develop into maturity, but they have higher growth rate than the diploid one. New tetraploids were produced by the retenhon of the second polar bodyfollowing fertilizahon (4N × 2N ♀), and they were found to be tetraploid, triploidand diploid by chromosome analysis (among 72 resultant fish, there were 19 tetraploid,32 triploid and 21 diploid, respechrely.)