ABSTRACTMuscle Type I collagen of five teleosts was characterized with respect to its thermal stability and subunit composition. The muscle collagen exhibited a higher denaturation temperature, Td, in solution and a higher degree of proline hydroxylation, compared with skin Type I collagen of the respective species. Moreover, the Td values seemed to increase with the increasing upper limit of environmental temperature of fish habitats. The subunit composition also varied with fish species; an α1α2α3 heterotrimer existed in eel and common mackerel and an (α1)2α2 heterotrimer in saury, while (α1)2α2 and α1α2α3 heterotrimers appeared to be major and minor components, respectively, in chum salmon and carp.