An assessment of the effects of arbuscular mycorrhizal (AM) infection on photosynthesis, carbon (C) allocation, translocation and biomass production of cucumber, grown in sand culture, was made using a previously determined phosphorus (P) supply (0·13 mol m−3 P) which had a significant impact on AM infection. Separation of a direct effect of AM infection from an indirect one due to an enhanced leaf P status was achieved using a comparable non-mycorrhizal treatment (NAM + P) supplemented with extra P (0·19 mol m−3 P). Total leaf P concentration, specific leaf mass, photosynthetic capacity, and incorporation of 14C into non-structural carbohydrate pools were dependent on leaf age. Both maximum and ambient photosynthetic rates were significantly higher in the youngest fully expanded leaves from AM and NAM + P plants which also had the higher leaf P concentrations. There were no differences in the total concentrations of starch, sucrose, raffinose or stachyose in young or old leaves among AM, non-mycorrhizal (NAM) and NAM + P treatments. However, younger leaves of NAM plants showed a shift in 14C-partitioning from stachyose and raffinose synthesis to starch accumulation. Determination of ADP-glucose pyrophosphorylase (AGPase), sucrose synthase and sucrose phosphate synthase enzyme activities revealed that only AGPase activity was correlated with the increased incorporation rate of 14C into starch in young leaves of NAM plants. Although there were significant AM-specific effects on C translocation to the root system, AM plants had similar rate of photosynthesis to NAM + P plants. These results suggest that the increase in photosynthetic rate in leaves of AM-infected cucumber was due to an increased P status, rather than a consequence of a mycorrhizal ‘sink’ for assimilates.