Although plant phosphate uptake is reduced by low soil temperature, arbuscular mycorrhizal (AM) fungi are responsible for P uptake in many plants. We investigated growth and carbon allocation of the AM fungus Glomus mosseae and a host plant (Plantago lanceolata) under reduced soil temperature. Plants were grown in compartmented microcosm units to determine the impact on both fungus and roots of a constant 2.7 degrees C reduction in soil temperature for 16 d. C allocation was measured using two 13CO2 pulse labels. Although root growth was reduced by cooling, AM colonization, growth and respiration of the extraradical mycelium (ERM) and allocation of assimilated 13C to the ERM were all unaffected; the frequency of arbuscules increased. In contrast, root respiration and 13C content and plant P and Zn content were all reduced by cooling. Cooling had less effect on N and K, and none on Ca and Mg content. The AM fungus G. mosseae was more able to sustain activity in cooled soil than were the roots of P. lanceolata, and so enhanced plant P content under a realistic degree of soil cooling that reduced plant growth. AM fungi may therefore be an effective means to promote plant nutrition under low soil temperatures.