A major collagenous component of octopus skin was isolated from limited pepsin digests by selective salt precipitation in acidic and neutral solvents. Chromatography on Cm-cellulose of the denatured collagen and sodium dodecyl sulfate-gel electrophoresis, agarose gel filtration, and chemical analysis of the chromatographic fractions revealed that two distinct alpha chains, alpha 1 and alpha 2, are present in a molar ratio of 2:1. The formaldehyde-cross-linked collagen yielded a large proportion of gamma chain, which was identified as gamma 112 by its chromatographic behavior and amino acid composition. Moreover, cyanogen bromide peptide mapping suggested a structural relationship between octopus skin collagen and calf skin Type I collagen. These composite results led us to the conclusion that the native collagen molecules are designated by the formula (alpha 1)2 alpha 2 and correspond to type I collagen in higher vertebrate tissues. On the basis of these findings, we assume that Type I or Type I-like collagen might have evolved along independent phylogenetic lines in protostomian and deuterostomian animals.