Hydrogen, which is a clean energy source, is one of the alternatives for fossil fuels. Biological hydrogen production is one of the hydrogen production methods. Rhodobacter capsulatus is a photosynthetic bacterium that produces hydrogen via photofermentation. R. capsulatus can also synthesize some valuable by-products such as Poly-beta- hydroxy butyric acid (PHB), which is a biodegradable bioplastic. In a two stage biohydrogen production system, which is combination of dark fermentation and photofermentation, dark fermentor effluents are used for photofermentation by R.capsulatus. Dark fermentor effluents usually contain high amount of acetate. High amount of acetate may decrease the efficiency of hydrogen production by causing high amount of PHB production. Therefore, it is significant to determine optimum acetate concentration for photofermentation. In this study, the effects of acetate concentration on hydrogen and PHB production by R.capsulatus were investigated by growing bacteria at various acetate concentrations (10 mM-65 mM). In addition, gene expression analysis was performed to investigate the effects of acetate at transcriptional level. For this purpose, expression levels of the genes that encode nitrogenase which is the enzyme that catalyzes hydrogen production and PHB synthase, which is the key enzyme of the PHB synthesis pathway, are examined. Optimum acetate concentration for photofermentation with high hydrogen yield and low PHB amount was determined to be in the range 25 mM-50 mM. nifD expression was found to be high at optimum acetate concentrations and phaC expression was found to be the highest at 65 mM.