Spawning biomass of chub mackerel Scomber japonicus was estimated for the Pacific subpopulation off central Japan in 1996 by a daily egg production method. The 451 samples of the adult mackerel used for estimating the biological parameters were collected for April to June in 1996 from the research takes and commercial catch in the waters around Izu-Islands. Spawning frequencies were 9%, 47% and 47% for the respective months. Batch fecundities for the 3 months were 31,097, 27,600 and 21,500 eggs/female. Spawning biomass was estimated to be 35,000 t for June when the spawning activity was highest among the 3 months.