A genomic bank has been constructed using DNA isolated from a dystrophic strain of chickens. The library was screened for myosin heavy chain (MHC) sequences using a cDNA probe and 11 positive recombinants isolated. Identity of the clones was confirmed by positive RNA selection via hybridization of the clones with muscle RNA and subsequent translation of the hybridizable RNA in vitro. Restriction maps indicate that the clones can be divided into 7 distinct groups; some of these groups do, however, share similar or homologous sequences. Hydridization of the clones to RNA derived from leg, breast, heart, and brain reveals that all of the clones code for the muscle-specific MHC isoforms. These experiments also show that there are at least 3 size classes of MHC mRNA sequences, whose relative amounts appear to be modulated in a tissue- and developmental stage-specific manner.