Polyhydroxybutyrate (PHB) was produced by Ralstonin eutropha DSM 11348 (formerly Alicaligenes eutrophus) in media containing 20-30 g l(-1) casein peptone or casamino acids as sole sources of nitrogen. In fermentations using media based on casein peptone, permanent growth up to a cell dry mass of 65 g l(-1) was observed. PHB accumulated in cells up to 60%-80% of dry weight. The lowest yields were found in media without any trace elements or with casamino acids added only. The residual cell dry masses were limited to 10-15 g l(-1) and did not contain PHB. The highest productivity amounted to 1.2 g PHB l(-1) h(-1). The mean molecular mass of the biopolymer was determined as 750 kDa. The proportion of polyhydroxyvalerate was less than 0.2% in PHB. The bioprocess was scaled up to a 300-1 plant. During a fermentation time of 39 h the cells accumulated PHB to 78% w/w. The productivity was 0.98 g PHB l(-1) h(1).