A glasshouse experiment was done to assess the development and phosphate metabolism of mycorrhizas formed by species of arbuscular mycorrhizal fungi (AMF) from two different genera, Gigaspora and Glomus on Desmodium ovalifolium plants at three concentrations of a phosphate source. The addition of phosphate (0–100 mg P kg−1) had no effect on the alkaline phosphatase activity, stained histochemically, in the intra-radical mycelium of Gigaspora rosea (BEG111), but decreased that of Glomus manihotis (BEG112) over a 10-wk period. The alkaline phosphatase activity of the extra-radical mycelium was unaffected by increasing phosphate addition (0–100 mg P kg−1) in both species of AMF over a 10-wk period. The extra-radical mycelium of Gi. rosea (BEG111) accumulated polyphosphate, determined by staining with 4′,6-diamidino-2-phenylindole, whereas polyphosphate was not detected in the extra-radical mycelium of G. manihotis (BEG112). This work indicates differences in the mechanisms of phosphate metabolism in the mycelium of AMF from different genera on a tropical host. This might be determined by the life-cycle strategies of these fungi, in particular the formation of auxiliary cells in Gigaspora. The possibility of a negative-feedback mechanism between alkaline phosphatase and polyphosphate in the extra-radical mycelium of Gi. rosea (BEG111) and the role of polyphosphate in the symbiosis are discussed.