A marine photosynthetic bacterium, Rhodopseudomonas sp. strain W-1S, accumulated polyhydroxybutyrate (PHB) to 56% of the dry cell weight under microaerobic and photoheterotrophic conditions. The addition of NaCl and phosphate was essential for the accumulation. Acetate was the best carbon source for PHB accumulation, whereas adding malate, succinate, or sugars that did not support the growth in this strain resulted in no accumulation. The strain accumulated PHB to about 50% of the dry cell weight at the logarithmic phase of growth. These results suggested that the accumulation would be growth associated under microaerobic and photoheterotrophic conditions. Vitamins, especially biotin, exerted an inhibitory effect on PHB accumulation and a stimulatory effect on pigment production. There might be regulation of carbon flux to PHB or pigments as storage materials.