In order to examine the functions of the red and white muscle of jack mackerel Trachurus japonicus during swimming locomotion, the fish was dissected to observe muscle distribution along the body. The electrical activity of the lateral muscle was analyzed using an electrophysiological technique. The ratio of red muscle to total muscle in a transverse section of the fish body reached a maximum at a position between 55% and 65% of fork length from the snout. The red and white muscles accounted for 5.1% and 61% of the body weight respectively. The white muscle was 12 times as heavy as the red muscle. In the electromyographic observations, the red muscle was active in sustained swimming at low speeds, whereas there was no electrical activity from the white muscle at speeds below 108cm/s for fish of 17.8cm in fork length. The bursts of discharge appearing in the electromyograph were measured for three components: frequency, amplitude, and duration. The frequency increased in proportion to swimming speed in both muscles. The tendency for the amplitude to increase with swimming speed was much more remarkable in the red muscle than in the white muscle. The duration in the red muscle decreased, whereas in the white muscle it rose with the swimming speed. The results indicate that for jack mackerel only the red muscle is used in sustained swimming, while the white muscle is used during burst swimming above a threshold speed of 6.4BL/s, together with red muscle.