Myosin is a fibrous protein that plays a role in muscle contraction by interacting with actin. Fish myosins are quite unstable compared with mammalian ones, despite of the similarity in protein structure. This is characterized by loss of ATPase activity, protein unfolding, and aggregation. The stability differs among fish species. The differences observed in the stability are reflected by the structural stability of myosin subfragments: subfragment 1 (S-1) and light meromyosin (LMM). Cold-water fish possess less stable myosin than warm-water species; the environmental temperatures at which fish live are related to the structural stability.