Decomposition and histamine formation were studied with fresh mahimahi ( Coryphaena hippurus ) incubated in seawater at 0, 10, 21, and 32°C. The rates of decomposition (loss of quality) and histamine formation both increased at warmer incubation temperatures. Spoilage bacteria were primarily psychrotrophic at 0 and 10°C, while mesophilic bacteria predominated at 21 and 32°C. An increase in pH of the loin tissue was correlated slightly with the histamine level. The correlation between histamine level and loss of quality, however, was high. Also there was a strong correlation between odor of the fillet and histamine production during spoilage. Prior frozen storage at -20°C inhibited the rate of subsequent histamine formation, but did not affect the extent of quality loss. Loss of histamine during cooking (baking or steaming) had no appreciable effect on the residual histamine level of spoiled fish. Seawater was a much more efficient heat transfer medium than air during incubation. The rates of histamine formation and loss of quality were significantly greater in seawater than in air at the same temperature.