Strains of Rhizobium spp. isolated from leguminous plants and standard strains accumulated 27% to 57% polyhydroxyalkanoate (PHA) of their cell biomass. Among these cultures, one strain of Rhizobium meliloti synthesized 10-30% more PHA than others and contained 3% hydroxyvalerate (HV) when grown on sucrose as carbon substrate. The occurrence of hydroxybutyrate (HB) and HV was confirmed by GC and H-1 NMR analysis. Treatment of the culture with 4'-N-piperidinobutyl-2-chlorophenoxazine resulted in a mutant which synthesized up to 69%, PHA of the cell biomass with an improved yield of 11 to 47% under different carbon and nitrogen ratios, compared to the parent strain.