The expression of dystrophin was examined immunocytochemically in skeletal and cardiac muscle of canine carriers of X-linked muscular dystrophy, a model of Duchenne muscular dystrophy. In skeletal muscle, the expression of dystrophin was heterogeneous, individual fibers being strongly positive, negative, or intermediate in staining. Some fibers expressed dystrophin discontinuously along their length. Between 4 and 24 weeks of age, the expression of dystrophin in skeletal muscle of carriers became more uniform and the number of negative fibers declined. In the heart dystrophin was also expressed in a mosaic pattern, individual cells being either fully positive or negative by immunocytochemistry. There was no apparent change in the pattern of expression between 12 weeks and 4 years of age. These results indicate that in the mosaic skeletal muscle fibers of carriers dystrophin is initially expressed in a heterogeneous pattern and that, in time, it becomes more uniformly distributed. This change could be explained by recruitment of competent satellite cells or by gradual diffusion of the protein.