Three halotolerant bacterial strains; Rhodobacter sphaeroides ES16 (the wild type) and the two mutant strains of R. sphaeroides ES16, namely N20 and U7, were cultivated in glutamate-malate (GM) medium and screened for production of polyhydroxybutyrate (PHB). The mutant strains N20 and U7 were found to accumulate PHB (53.9 and 42.0% of DCW, respectively) 3.6 and 2.8 times higher than the wild type strain (19.5% of DCW), respectively. R. sphaeroides N20 were selected for studies on the effects of nutrient and environmental conditions on PHB accumulation. The optimal condition was 4 g/l acetate, 0.02 g/l (NH4)2SO4, C/N ratio of 6:1, 1.0 g/l K2HPO4, 1.0 g/l KH2PO4 and 3% NaCl with initial pH at 7.0. Under this optimal condition, the maximum PHB accumulation increased from 53.9% to 88% of DCW and 9.11 ± 0.08 g/l biomass, 8.02 ± 0.10 g/l PHB concentration were achieved after 60 hrs cultivation at 37ºC. These results are the highest values ever obtained from photosynthetic bacteria reported so far.