We have recently determined the complete nucleotide sequences of the cardiac alpha- and beta-myosin heavy chain (MyHC) genes from both human and Syrian hamster. These genomic sequence data were used to study the molecular evolution of the cardiac MyHC genes. Between the alpha- and beta-MyHC genes, multiple gene conversion events were detected by (1) maximum parsimony tree analyses, (2) synonymous substitution analyses, and (3) detection of pairwise identity of intron sequences. Approximately half of the 40 cardiac MyHC exons have undergone concerted evolution through the process of gene conversion with the other half undergoing divergent evolution. Gene conversion occurred more often in exons encoding the alpha-helical myosin rod domain than in the globular head domain, and an apparent directional bias was also observed, with transfer of genetic material occurring more often from beta to alpha.