Nutrient limitation conditions, optimization and comparison of polyhydroxyalkanoate (PHA) yields and biomass production by parent and mutant strains of Rhizobium meliloti were investigated. Complex interactions among concentrations of sucrose (5-55 g/l), urea (0.05-0.65 g/l) inoculum (10-250 ml/l) and K2HPO4 (0.5-2 g/l), were studied using central composite rotatable design (CCRD) experiments. Phosphate-limiting medium (0.33 g K2HPO4/l) in the presence of excess carbon (sucrose 42.5 g/l) results in more production of PHA (2.2 g/l) in the parent strain. In comparison, the mutant strain required moderate levels of sucrose (30 g/l), along with excess of phosphate (1 g/l) for high PHA content of cell biomass (80%) and PHA yield (3.3 g/l). Optimised PHA production (biomass 4.8 g/l and PHA 3.09 g/l) by the parent strain occurred at: sucrose 51.58 g/l, urea 0.65 g/l, K2HPO4 0.48 g/l and inoculum 10 ml/l. In the mutant strain, higher yields of biomass (9.05 g/l) and PHA (5.66 g/l) were obtained in Optimised medium containing: sucrose 55 g/l, urea 0.65 g/l, KHPO4 1.0 g/l and inoculum 150.58 ml/l. (C) 2003 Elsevier Ltd. All rights reserved.