Yeast Saccharomyces cerevisiae IFO 2114 was cultured in 25l of 75% sea water containing liquid obtained from mackerel waste juice and molasses (WY) for 48h, and (1) nitrogen and pho-sphorus contents in WY medium and (2) proximate and fatty acid compositions of the harvested yeast (WY yeast) were determined. The amount of harvested WY yeast was sufficient to feed rotifers (150×10⁶ indiv./ton) for a day. Furthermore, the yeast reduced nitrogen and phosphorus contents in the medium to a level which was considered to be safe to discard. On the other hand, WY yeast was higher than yeast cultured in MY medium and baker's yeast and far lower than ω yeast and marine chlorella in lipid content and percentage of ω3 HUFA in total fatty acids.