The loss of vitamin B₁₂ in round herring meats during various cooking treatments was evaluated. Although amounts of vitamin B₁₂ were three times greater in the viscera (37.5±10.6 μg/100 g fresh weight) than in the meats, about 73% of total vitamin B₁₂ found in the whole fish body (except for head and bones) were recovered in the meats (5.1±1.0 μg of vitamin B₁₂). The vitamin B₁₂ contents of the round herring's meats were significantly decreased up to ~62% during cooking by grilling, boiling, frying, steaming, and microwaving. There was, however, no loss of vitamin B₁₂ during vacuum-packed pouch cooking. Model experiment using hydroxocobalamin suggest that loss of vitamin B₁₂ is dependent on the degree of temperature and time used in conventional cooking, and is further affected by the concomitant ingredients of food. Retention of vitamin B₁₂ was not dependent on vacuum or temperature (or both) used in the vacuum-packed pouch cooking.