The sequence corresponding to approximately 98% of mouse skeletal muscle actin mRNA was determined from cDNA clones isolated from a library of recombinants in pBR322. One of these clones contains DNA corresponding to the complete amino acid coding region and a large part of the 5' and 3' untranslated regions of the mRNA. Comparison of the mouse coding region (conserved at the amino acid level) and noncoding regions with the corresponding regions of the rat skeletal muscle actin gene indicates that the noncoding regions have also been under selective pressure during evolution.