Specimens of horse mackerel were killed by three different procedures, i.e. stabbing in the spinal cord (instant killing), dipping in cold sea water (temperature shock), and leaving to die in the air (struggled killing). After death the fish were stored at 0 and 8°C. In the fish of the struggled killing group, the rates of decrease in rigor index and of breaking strength of dorsal muscle were fastest. An evident difference was observbed in the changing rates of concentrations of ATP, IMP, creatine phosphate, and lactic acid among the three killing procedures. All the rates were slowest in the instantly killed group. When full rigor was observed, ATP was still contained in significant amounts. The rate in progress of rigor mortis was almost comparable with an increase in lactic acid level. The storage temperature scarcely affected the progress of rigor mortis, but did affect the rate of changes in ATP and K value. From these results, it was suggested that stabbing of the spinal cord was the best procedure to delay the fall in quality of horse mackerel.