When mycorrhizal rootlets of beech are aged for several days in water, their respiration rate falls. If phosphate is applied, the respiratory rate rises and the rise in oxygen consumption is proportional to phosphate uptake in replicate samples aged for equivalent times. The ratio of μg-atom phosphorus to μg-atom oxygen absorbed is usually around or just below unity, but it varies with the state of the tissues to values as low as 0·28. High values of this ratio in the later state of ageing may be spurious because, in senescent mycorrhizas, phosphate penetrates non-metabolically. High values in fresh mycorrhizas occur because the rate of phosphate-induced oxygen uptake is low but it rises faster than that of phosphate uptake in the early stages of ageing. The conclusion is drawn that the processes of phosphate absorption and polyphosphate formation are often 20 to 30% efficient with respect to respiratory oxidative phosphorylation. The linear relationship of total oxygen consumption to phosphate absorption observed in fresh mycorrhizas is due to the pre-emption of a portion of total phosphorylation by polyphosphate production.