Cofilin is an actin-modulating protein of 20 kDa, which is widely distributed throughout muscle and non-muscle cells. By means of immunoblotting combined with two-dimensional gel electrophoresis, we found that two cofilin variants, muscle type (M-type) and non-muscle type (NM-type), exist in mammals, while a single isoform exists in chickens. During in vitro myogenesis of mouse C2 cells, expression of the M-type cofilin was upregulated. To better understand the nature of the M-type cofilin, we cloned cDNAs encoding M-type cofilin from the cDNA library of C2 myotubes and determined the entire sequence. The deduced peptide sequence contained a nuclear localization signal and a putative actin-binding sequence as reported in NM-type cofilin. The sequence showed 81% identity in the amino acid residues with the mouse NM-type cofilin sequence and, interestingly, higher homology (96% identity) with that of chicken cofilin. The mRNA encoding M-type cofilin, though it contains two variants that differ in the size of their 3'-non-coding sequences, was detected predominantly in heart, skeletal muscle, C2 myotubes, and testis by Northern blotting, while the mRNA for NM-type cofilin was seen in a variety of non-muscle tissues. The presence of the muscle type isoform of cofilin strongly suggests that cofilin is deeply involved in the regulation of actin function not only in non-muscle cells but also in muscle cells.