The bigeye tuna steaks (ca. 8×6×2 cm), after being treated with carbon monoxide (CO) gas for 5 days, were stored together with the untreated control fish steaks under two different conditions, in ice at 0℃ for 7 days and frozen stored at -20℃ for 6 months. For each storage condition, the changes in metmyoglobin formation (metMb%), color appearance and CO residue in the surface layer (2 mm thick), the middle layer (2~4 mm from surface) and the inner layer (4~6 mm from surface) of the CO gas-treated and the control were measured and compared. The results revealed that, during the 7-day ice storage period, the metMb% values in the three layers of the CO gas-treated tuna steak increased gradually, but all remained below 10%. For the control, the metMb% in the middle and inner layers were as high as 50% before storage, obviously being higher than that of the surface layer, but increased only slowly in all three layers throughout the iced storage.

　
   In views of the measured tristimulus color values (Hunter L, a and b) among the three layers of tuna steaks, the +a value of the CO gas-treated steak was lower than that of the control. The amount of the CO residue in the CO gas-treated steak decreased drastically during iced storage and became undetectable in both the middle and the inner layers after 7 days of storage.

　
   In the case of frozen storage at -20℃ for 6 months, the values of metMb% and Hunter a for CO gas-treated steak remained almost constant, whereas the value of metMb% of control increased gradually up to 40~60% while the +a value decreased from 11 to 5. The CO residue in the surface and middle layers decreased during the early stage of the six-month freezing storage followed by no apparent change. There was no apparent variation in CO residue in the inner layer of tuna steak throughout the frozen storage.