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 (NH 4 ) 2 SO 4 , C/N ratio of 6:1, 1.0 g/l K 2 HPO 4 , 1.0 g/l KH 2 PO 4 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.Polyhydroxyalkanoates (PHAs) are a class of natural *Corresponding author polyesters, which can be produced and accumulated by many Gram-positive and Gram-negative bacteria from at least 75 different genera. These polymers are accumulated intracellularly under conditions of nutrient stress and act as a carbon and energy reserve (Steinbüchel, 2001;Reddy et al. 2003). PHB is the well known studied and best characterized PHAs. It is accumulated when carbon and energy sources are in excess, but growth is limited by the lack of oxygen, nitrogen, or phosphorus source. The characteristic of this compound is similar to synthetic plastic or petrochemical-based plastics (such as polypropylene, polyurethane, vinyl chloride and hexachloroethane etc.). Therefore, PHB and its copolymer can be used as biodegradable plastic, which can reduce the current problems with decreasing fossil resources and environmental impact caused by plastic garbage (Luengo et al. 2003). In addition, it has a promising application in medicine, material science and agriculture, etc. (He et al. 2004). However, the important factor preventing the industrial and commercial production of PHB is its high price of production as compare to synthetic plastic. Therefore, improved cultivation medium and conditions are Sangkharak, K. and Prasertsan, P.2 required for reducing the cost (Khanna and Srivastava, 2004).Halotolerant bacteria were reported to produce high amounts of PHAs (40-60% DCW) were accumulated in halotolerant bacteria under the starvation condition (Hassan et al. 1997;Khatipov et al. 1998;Luengo et al. 2003;Chen et al. 2006). Recently, the highest PHAs production was obtained from R. sphaeroides strain 14F which showed (3.5 g/l PHA, 60% DCW) cultivated in modified GM medium where malate was substituted by 5 g/l fructose under twostage aerobic dark condition (Lorrungruang et al. 2006). Halotolerant photosynthetic bacteria have the advantage over the other microorganisms in their ability to adjust themselves to both presence and absence of photo as...