SummaryAs the fact that feedingSpirulinacan improve carcass uality of fish, this study describes the effect on muscle protein in red sea bream,Pagrus major. The fish were fed on a diet supplemented with 2%Spirulinafor 95 days under commercial rearing conditions.Spirulinafeeding elevated protein synthesis, which was estimated by muscle RNA/DNA and RNA/protein ratios. Muscle protein was fractionated into sarcoplasmic, myofibrillar, alkali‐soluble and stromal fractions. The stromal fraction was significantly higher in theSpirulina‐ fed group. The proportion of the fractions approached that of wild fish somewhat bySpirulina‐ feeding. The results possibly indicate thatSpirulinacontributes to elevate protein assimilation and increase stromal fra