The cultural and spawning experiments of jack mackerel Trachurus japonicus (TEMMINCK et SCHLEGEL) from our hatchery were carried out from April 18, 1980 to July 16, 1981. The growth of the cultured fish was considerably high and the fish reached 17.4cm in fork length and 67.0g in weight in average at the end of its first year. The optimal temperature for the growth is ascertained to be between 18° and 26°C. All males are found to be spermiating without hormone injection in April and June, 1981. In some females the ovaries are well enlarged, with maturing eggs deveploped to primary or tertiary yolk stage in May and June, 1981. It is suggested that the cultured fish attained sexual maturity at least at a length of about 15cm in fork length. Twenty six females and 25 males are treated with gonadotropin in May 26, 1981, and placed in a circular tank of about 6 ton capacity at the beginning of the spawning experiment. At the water temperature of about 21°C, spawning began at 32:30 hr, and completed at 39:45 hr after receiving gonadotropin. The fertilized eggs incubated at the water temperature of abuot 21°C began hatching at about 34:40 hr after first spawning.