A method is described for extraction of alpha-paramyosin in amounts comparable to that formerly attained for beta-paramyosin (15-25 mg/g of muscle). A modification of the procedure for sodium dodecyl sulfate gel electrophoresis is described that permits the separation on coelectrophoresis of alpha-paramyosin (207 000 daltons) and beta-paramyosin (200 000 daltons). The alpha- and beta-paramyosins also can be distinguished by gel electrophoresis at pH 2.3 and by differences in solubility in the region of 0.2-0.4 ionic strength at neutral pH. Evidence is presented that the segment lost from alpha-paramyosin during degradation to beta-paramyosin came from the C-terminal end. This evidence is based on determinations of N- and C-terminal amino acids and on the size of segments obtained after chemical cleavage at the sites of Cys residues. It has been observed earlier that the solubility characteristics of beta-paramyosin at neutral pH are determined by the C-terminal one-third of the molecule and the present results indicate that the additional small segment of about 3.5% of the total mass that is present in the C-terminal end of alpha-paramyosin accounts for the marked difference in solubility of the two forms.