We have isolated four unique human sarcomeric myosin heavy chain (MHC) genomic clones using rat MHC cDNA clones as probes. Three of these clones contain adult skeletal muscle-specific DNA sequences, whereas one clone contains embryonic skeletal muscle-specific sequences. This developmental and tissue specificity was determined by hybridization of each of the human clones to MHC mRNA from different muscle tissues. Cross-hybridization studies indicate that certain sequences of the human MHC genes have been conserved through evolution while other portions have diverged considerably. Preliminary evidence demonstrates that the MHC gene family is polymorphic in human populations. Each of the human MHC genes was shown to have repetitive sequences in multiple positions, both within the gene and in adjacent flanking DNA sequences. We have shown that, in contrast, four rat MHC genes have far fewer repetitive sequences even though two of the four genes contain the same muscle specificity as the human genes. Therefore, these genes may be useful to study gene evolution and repetitive sequence transposition.