The composition of muscle proteins and free amino acids in the mudskipper (hibernant fish) were investigated before and after hibernation (maturity period: August, hibernation period: November thru. April). It was found that crude proteins were 17.6% in August, 17.5% in November and 16.9% in April, while among the muscle proteins, sarcoplasmic proteins were constituted up to 19.2～20.4%, 58.8～61.3% for myofibrilla proteins, 11.2～13.2% for intracellular proteins and 7.5～8.3% for stroma proteins. Composition changes of the muscle proteins were hardly noted until November but during the hibernation(from Nov. to Apr.) the amount of the sarcoplasmic proteins and the myofibrillar proteins decreased pronouncedly. As for the sarcoplasmic proteins, 14 subunits were found and among them, the amount of 30 kDa and 46 kDa subunits were found to increase slightly in April compared with those in November, while the amount of 35 kDa and 65 kDa subunits were decreased slightly. As for the myofibrilla proteins, 13 subunits were found and detectable changes in their composition were not observed until November but in April the amount of myosin heavy chain was increased by 3%, while the amount of actin decreased by 3% when those are compared with the results in November. The composition of amino acids in the muscle proteins was hardly changed during the observation period. But there were considerable changes of composition of free amino acids. Glycine and alanine were found to be the major free amino acids. The most striking feature was the changes in the glycine and arginine content: the former, which is a dominant free amino acid, was increased by two-fold in April compared with that in August and the latter was increased by two-fold in November and by four-fold in April. It was also found that the amount of essential amino acids (i.e., lysine and histidine) and others (alanine, glutamic acid, serine, aspartic acid and valine) increased significantly during the hibernation period.