The interactive effect of phosphate-solubilizing bacteria and arbuscular mycorrhizal (AM) fungi on plant use of soil P sources of low bioavailability (endogenous or added as rock phosphate [RP] material) was evaluated by using soil microcosms which integrated P-32 isotopic dilution techniques. The microbial inocula consisted of the AM fungus Glomus intraradices and two phosphate-solubilizing rhizobacterial isolates: Enterobacter sp, and Bacillus subtilis, These rhizobacteria behaved as ''mycorrhiza helper bacteria'' promoting establishment of both the indigenous and the introduced AM endophytes despite a gradual decrease in bacterial population size, which dropped from 10(7) at planting to 10(3) CFU g(-1) of dry rhizosphere soil at harvest. Dual inoculation with G. intraradices and B. subtilis significantly increased biomass and N and P accumulation in plant tissues. Regardless of the rhizobacterium strain and of the addition of RP, AM: plants displayed lower specific activity (P-32/P-31) than their comparable controls, suggesting that the plants used P sources not available in their absence, The inoculated rhizobacteria may have released phosphate ions (P-31), either from the added RP or from the less-available indigenous P sources, which were effectively taken up by the external AM mycelium, Soluble Ca deficiency in the test soil may have benefited P solubilization. At least 75% of the P in dually inoculated plants derived from the added RP, It appears that these mycorrhizosphere interactions between bacterial and fungal plant associates contributed to the biogeochemical P cycling, thus promoting a sustainable nutrient supply to plants.