Comparative growth profiles were investigated for strain difference in 8 strains of the guppy Poecilia reticulata. There was an obvious difference in body length between female and male from Day 60, which became more pronounced with growth to Day 180. Differences in body length between strains were found significantly at each age. Using strain comparison, the heritability of body length at various ages was estimated, and this heritability changed with age. The change of heritability suggests different genetic control in three stages for the growth curve. They are maternal genotypic effects on the body size at birth, growth genes in the growing stage, and an inhibitory gene influencing final body size.