cDNA clones of chick muscle creatine kinase mRNA were prepared by conventional procedures and then extended by primer extension to include the entire coding region of the mRNA. The nucleotide sequence of the cDNA clones permits, for the first time, determination of the complete sequence of the 381 amino acids comprising the muscle creatine kinase subunit of a higher vertebrate. In addition, available information regarding amino acid residues, intergral or proximal to the active site, allows tentative positioning of part of the active site within the primary sequence. Comparison of the chick muscle creatine kinase amino acid sequence with partial amino acid sequence for rat and rabbit muscle creatine kinase indicates that the primary structure of this enzyme is strongly conserved in evolution.