Yearly changes of concentration of nitrate and phosphate in Tomari, Hokkaido, Japan, from 1986 to 2001, were analyzed. Mean concentration of nitrate at 0m depth was 2.2±1.6SD (range, 0.3-6.1) μM. Gametophytes of Laminaria religiosa were cultured for 29 days in different nitrate concentrations (+0.0, +1.0, +2.0, +3.0, +4.0, +5.0, +6.0μM) in order to examine the effects of nitrate on the growth of gamethophytes in batch culture. Cell number of the gamethophytes increased at concentrations of more than +4.0μM nitrate. To observe the effects of deep seawater on growth of gamethophytes of L. religiosa, the gamethophytes were cultured in deep seawater at Otaru, Hokkaido, Japan, (concentration of nitrate, 15.8 μM) and in surface seawater at Yoichi. Hokkaido, (mean concentration of nitrate, 4.4μM) under laboratory conditions. After 29 days from start of batch culture, cell number in the deep seawater was 2.6 times of the cell number in surface seawater.