We have used a myosin heavy chain gene fragment to probe two chicken genomic libraries. The fragment, derived from a gene expressed in the fast-white fibers of adult chickens, contains 1400 bases upstream from the translational start site and 1600 bases downstream from the initiation codon. Thirty-one unique nonoverlapping clones were isolated. All of the genes showed homologies in the nucleotide sequences which code for the globular head portion of the myosin protein while no extensive homologies were detected in the 5'-flanking sequences. The relationships between the genes were studied using oligomeric sequences as probes. The hybridization patterns showed that seven of the genes fall within a well defined subgroup. The exon containing a domain of the nucleotide (ATP) binding site was sequenced for all seven of these genes and shown to be, except for 2 nucleotides in one of the genes, completely conserved. The lack of sequence conservation in the 5'-nontranslated portions of the genes was exploited in the preparation of transcript-specific probes. We have used these probes to show that two of the isoforms are expressed in a tissue-specific and developmental stage-specific manner in the chicken.