The stimulating effect of strobe light on jack mackerel Trachurus japonicus was examined for the heart rate change under different flash frequencies. Heart rate change was clearly observed to increase during strobe flashing, with a tendency of sudden decrease immediately after ending of the stimulus. The observation was repeated three times for each individual with an interval of two hours. The data analysis focused on the stimulating effect of the strobe light with flash frequencies ranging from 1.7 ?? 430Hz, as well as habituation by repeated trials. Under these test conditions, the flash frequencies of 5 and 10Hz produced the strongest effect of strobe stimulus. There was no significant decrease of the stimulating effect by the repeated trials, suggesting that of habituation does not occur.