ATPase activities of molluscan adductor muscle myosins show both muscle and species specific differences: ATPase activity of catch muscle myosin is lower than that of phasic muscle myosin; a 4-5-fold difference exists between the activities of phasic striated muscle myosins from the bay scallop (Argopecten irradians) and sea scallop (Placopecten magellanicus). To characterize the light chains of these myosins we determined the cDNA sequences of the essential light chains and the regulatory light chains from Placopecten striated and catch muscle. The nucleotide sequences of the essential light chains from Placopecten striated and catch muscle myosins are identical and show 94% identity and 98% homology to the Argopecten essential light chain indicating that the tissue and species specific differences in ATPase activities are not due to the essential light chain. We identified three regulatory light chain isoforms, one from striated and two from catch muscle. Sequence differences were restricted to nucleotides encoding some of the N-terminal 52 amino acids. The three recombinant Placopecten regulatory light chain isoforms and the Argopecten regulatory light chain were incorporated into hybrid myosins that contained the essential light chain and heavy chain from Placopecten striated, Placopecten catch, or Argopecten striated muscle. Measurement of the ATPase activities of these hybrids indicates clearly that it is the myosin heavy chain and not the regulatory light chains that are responsible for the muscle and species specific differences in enzymatic activities. Analysis of genomic DNA indicated that these regulatory light chain isoforms are products of a single regulatory light chain gene that is alternatively spliced in the 5' region only.