Individual arbuscular mycorrhizal fungi can differ markedly in their ability to improve the phosphate nutrition and growth of host plants. In particular, Scutellospora calospora is relatively ineffective with some hosts and a growth depression is often observed. We have examined the abilities of S. calospora and Glomus sp. ‘City Beach’ strain WUM 16, grown in soils which promote extensive mycorrhizal colonization, to transfer phosphate (P) to Allium porrum. Phosphate uptake from the low-P soils (P0) was compared with uptake from soils amended with extra P (P1). In order to relate P transfer to physiological characteristics of the two fungi, inflow of P via fungus to the plant was combined with the surface areas of intercellular hyphae and arbuscules (symbiotic interfaces) to calculate the amount of P transferred per unit area of interface (P fluxes). ‘Hyphal inflows’ and ‘hyphal fluxes’ were also calculated on the assumption that P uptake by the plant was the same in mycorrhizal and equivalent non-mycorrhizal (NM) plants (the validity of this assumption is discussed). With both soil P treatments, P was taken up by plants colonized by both mycorrhizal fungi to a greater extent than by the equivalent NM controls. Hyphal inflows to plants in P0 soil that were colonized by Glomus sp. ‘City Beach’ were greatest from 14–21 d and decreased at later harvests. With P1 soil the inflow via Glomus sp. ‘City Beach’ peaked at a very high value at 21–28 d. Hyphal inflows into plants in P0 soil that were colonized by S. calospora increased throughout the experiment, and with P1 soil they remained steady at high values. With both fungi, the surface areas of the symbiotic interfaces increased greatly after 14 d, and generally there was little effect of higher soil P. With P0 soil, Glomus sp. ‘City Beach’ showed no significant differences in the ratio of surface areas of the two interfaces over the course of the experiment. Scutellospora calospora tended to produce a lower percentage of interfacial area contributed by arbuscules. With the mycorrhizal plants growing in their respective soils, there appeared no consistent differences between the two fungi with respect to fluxes of P across the interfaces. With P0 soil, fluxes via Glomus sp. ‘City Beach’ were initially higher than those via S. calospora, but later they were higher with S. calospora. With P1 soil the only difference was the 28–42-d period, when Glomus sp. ‘City Beach’ produced the higher flux (reflecting the low surface area at the time). The results show that relative inefficiency of S. calospora in its ability to transfer P, as reported by others, may result from different environmental conditions, use of different hosts, or even of different fungal isolates (strains).