The myosin content of the avian posterior latissimus dorsi muscle, a small fast-twitch muscle similar in fibre type to the much-studied pectoralis major muscle (type IIB), has been explored using high resolution chromatography of the proteolytic fragment known as subfragment-1 and of the products of its limited tryptic digestion, followed by N-terminal sequencing of selected peptides. The complexity of species found greatly exceeds that anticipated from the fibre-type homogeneity of the muscle and from previous studies (Bandman et al., Cell 29 (1982) 645-50; Lowey et al., J. Musc. Res. Cell Motility 4 (1983) 695-716; Crow & Stockdale Dev. Biol. 118 (1986) 333-42). A minimum of four heavy chain species were identified. One form, approximately 40% of the heavy chain complement, appears to be identical to the well-characterized type IIB isoform of the pectoralis major muscle. The remaining species differ from the pectoralis major form in primary sequence. None is identical to the post-hatch isoform of the pectoralis major muscle.