The effects of different nitrogen and carbon sources on cell growth, pH, and exopolysaccharide (EPS) and poly-(beta)-hydroxybutyrate (PHB) production by two strains of Rhizobium meliloti (M5N1 and Su47) are reported. Differences in the behavior of glucose- and fructose-grown cells were shown, in particular with the M5N1 strain. Growth in a glucose-containing medium was accompanied by acidification of the culture medium, which leads to cell death. On fructose, acidification was detected only in the medium with a mineral nitrogen supply. A lag phase in EPS production was observed with cells grown with glucose, probably related to an initial extracellular conversion of the carbohydrate into an acid. No lag phase was observed in EPS production from fructose or in PHB synthesis whatever the carbon source. A decrease in PHB content was noticed for both strains under conditions where acidification of media occurred. The extent of production, emphasized by the use of a coproduction index, indicates that the M5N1 strain is a more promising organism than is the Su47 strain for polymer production. Such a strain, put in rich medium (containing yeast extract) supplemented with fructose, accumulated PHB up to 85% of dry cell weight and excreted about 1.5 g of EPS per liter in the medium. Regulation of the coproduction of EPS and PHB by these cells is suggested.