The effect of phosphorus supply on concentrations of soluble carbohydrate within roots and on the development of mycorrhizas on subterranean clover was examined in two glasshouse experiments. Where increasing phosphorus supply decreased the percentage of root length converted to mycorrhizas, it also decreased the concentrations of soluble carbohydrates within roots. Shading, defoliation and low root temperatures also decreased both the percentage of root length infected and concentrations of soluble carbohydrate within roots. The percentage of root length infected was closely correlated with concentrations of soluble carbohydrates within roots irrespective of the treatment used to vary these concentrations. Small additions of phosphorus to severely deficient plants increased the percentage root length infected possibly by stimulating the growth of the mycorrhizal fungus.