Abstract. Satellite cells cultured from dystrophic (mdx) and from control mouse hindlimb muscles grow and fuse to form muscle fibers within 4–5 days. Total cell number and muscle-fiber formation are stimulated by bovine fibroblast growth factor (FGF). At low FGF levels (0.02–0.20 ng/ml) control satellite cells as well as fibroblasts are unresponsive, while mdx satellite cells show three- to four-fold increases in growth. Control cells do not begin to respond until FGF levels reach 1–5 ng/ml. Heparin, a major constituent of muscle fiber basal lamina, inhibits myogenesis in these mouse muscle cultures. The heightened sensitivity of mdx satellite cells to FGF may permit high rates of new fiber formation in vivo without a parallel hyperplasia in the muscle fibroblast population. This finding may be important in explaining successful regeneration in mdx muscle in vivo and the fact that mdx animals escape the catastrophic symptoms seen in the related human Duchenne muscular dystrophy.