In order to study developmental transitions of myosin heavy chain gene expression, we have cloned from newborn mouse skeletal muscle a recombinant plasmid (plasmid MHC 32) that contains an insertion coding for the COOH-terminal portion of an adult fast myosin heavy chain isoform of mouse skeletal muscle. By Northern blots and dot blots, it has been shown that the MHC 32 sequence reveals a broad cross-hybridization with RNA from different mammalian striated muscle tissues. Southern blots with mouse genomic DNA show only one homologous gene, but cross-hybridization at lower stringency to seven to eight different bands, some containing multiple genomic fragments, among which are probably the genes encoding the different striated muscle isoforms. S1 protection experiments with RNA from mouse skeletal muscle before and after birth demonstrate that plasmid MHC 32 is homologous to a major mRNA species of adult skeletal muscle. This adult mRNA is a predominant sequence within 5-6 days after birth. It begins to accumulate at 1-3 days; at the 18th day fetal stage, another major mRNA species is detected as partially homologous with the adult MHC 32 sequence. This fetal myosin heavy chain mRNA is still predominant at 1-3 days after birth, but is rapidly (by 5-6 days) replaced by the adult MHC sequence. There is thus a rapid transition after birth from fetal to adult skeletal muscle myosin heavy chain mRNA sequences.