(.) In this study we investigated the effects of temperature on fungal growth and tested whether the differences in fungal growth were related to the effects of temperature on carbon movement to, or within, the fungus. (.) Growth curves and C uptake-transfer-translocation measurements were obtained for three arbuscular mycorrhizal fungi (AMF) isolates cultured within a 6-30 degrees C temperature range. A series of experiments with a model fungal isolate, Glomus intraradices, was used to examine the effects of temperature on lipid body and 33 P movement, and to investigate the role of acclimation and incubation time. (.) Temperature effects on AMF growth were both direct and indirect because, despite clear independent root and AMF growth responses in some cases, the uptake and translocation of 13 C was also affected within the temperature range tested. Root C uptake and, to a lesser extent, C translocation in the fungus, were reduced by low temperatures (< 18 C). Uptake and translocation of 33 P by fungal hyphae were, by contrast, similar between 10 and 25 C. (.) We conclude that temperature, between 6 and 18 C, reduces AMF growth, and that C movement to the fungus is involved in this response.