The extent to which muscle fiber composition was determined by genes transmitted from parents and the correlated response of the synergistic muscle were analysed by using successive selection of rats. The foundation population (G0) was prepared by random choice from heterogeneous stock produced by random mating of three strains, Wistar-Imamichi, Fischer 344, and Donryu. Selective mating for a high percentage of fast-twitch fibers (%FT) in the deep portion of the lateral head of the gastrocnemius muscle and random mating were made from G0 to the seventh generation (G7). Successive selection caused a significant increase (P < 0.05) of %FT in the gastrocnemius muscle. The realized heritability for seven generations was 0.29. %FT in the soleus muscle of the selected line also increased (P < 0.05). In addition, significantly positive correlations (P < 0.05) were found between %FT in the gastrocnemius and soleus muscles in G1-G7. We conclude that approximately 29% of the variation of %FT was determined by genes transmitted from parents, and %FT in the gastrocnemius muscle was positively correlated with that of the soleus muscle.