Rhodobacter sphaeroides O.U.001 is able to produce hydrogen anaerobically upon illumination. The cells were screened for the presence of valuable by-products such as poly-e-hydroxy (PHB) butyric acid aiming to improve the feasibility of the system. Also waste water from a sugar refinery was used for bacterial growth to further increase the feasibility. Under aerobic conditions the standard growth media containing L-malic acid and sodium glutamate in 7.5/10 and 15/2 molar ratios and a medium containing 30% waste water from sugar refinery were used, In this case the maximum concentration of PHB produced were approximately 0.2 g l(-1) in both of the standard media whereas it was 0.3 g l(-1) in medium containing 30% waste water. By using the medium containing 30% waste water, PHB and hydrogen productions were determined under anaerobic conditions. The maximum concentration of PHB produced was around 0.5 g l(-1) and the amount of gas collected was 35 mi in 108 h. From these results it can be concluded that PHB can be collected during hydrogen production. The use of waste water from sugar refinery increased the yield. (C) 1999 Elsevier Science B.V. All rights reserved.