To elucidate genetic control for the growth of the guppy, size differences between Fancy (F) and Standard (S3) strains and between dwarf and normal in the F strain were examined. The two guppy strains, F and S3, displayed significant differences in growth. The F strain is bigger than the S3 strain in body length at Day 60. Using strain comparison, the heritability (Va/Vp) was estimated at 59. 2% in females and 38. 6% in males. Thus, the strain difference indicates the proportion of genetic determination of the character. Dwarf guppies were found in some of the inbred litter of brother-sister mating of the F strain. An overlap of distributions for body length at Day 60 was observed between F and S3 strains but not between dwarf and normal. From the comparison between dwarf and normal, the heritability was estimated at 88. 0 % to normal females and 84 5 % to normal males. These high values suggest that the dwarf is induced by a single gene with major effects on what is ordinarily growth. A single gene inheritance was supported by the segregation of the dwarf in some of the litter of inbreeding pedigree.