The effect of magnesium and phosphate limitation on the molecular weight distribution of poly-beta-hydroxybutyrate (PHB) in Alcaligenes eutrophus in continuous culture has been studied. Conditions of nitrogen limitation both with glucose excess (above ca. 20 g/L) and without excess were investigated. Under N-limitation and glucose excess, M(w) decreases when the magnesium content is decreased below 50% (19.7 mg/L) of the basal medium content; this also results in a broadening of molecular weight distribution (M(w)/M(n)) from 2 to 5 and a decrease in M(w) from 2 x 10(6) to 0.9 X 10(6). Below 20% of the basal content of magnesium (7.9 mg/L) these two trends were reversed. This behaviour was not observed in the absence of glucose excess. Under N-limitation and glucose excess, phosphate had virtually no effect on PHB M(w) or its distribution, whereas with no (or little) glucose excess M(w) of the PHB decreased with phosphate concentrations below 50% of the basal level (0.705 g/L). Hence, in continuous or fed-batch cultures, in addition to nitrogen limitation to allow for PHB accumulation, it is necessary to control both the addition of glucose (no excess) and also to maintain magnesium limitation (ca. 25% of basal medium level, 9.9 mg/L) and phosphate above 50% of the basal level (0.705 g/L). Thus, when broadening of molecular weight distribution (increase in M(w)/M(n)) is observed at the end of fed-batch culture it is probably caused by phosphate limitation and/or glucose excess.