An important function of the ectomycorrhizal sheath may be the storage and delayed translocation of nutrients to the host. In order to determine whether there are such phases, we quantified seasonal change in phosphorus content in sheaths of Cenococcum geophilum–Pinus strobus (white pine) ectomycorrhizae. Our field site was a mixed deciduous forest in northern, lower Michigan. White pine needle and root growth, and density of C. geophilum sclerotia were determined biweekly, as were buried meshes that served to index hyphal growth. Phosphorus content of sheaths of ectomycorrhizae was measured by energy-dispersive X-ray analysis of planed, frozen material. From May to Jun 1994, sheath P content declined 0.9%/d. From Jul to Oct sheath P content was low and varied little; C. geophilum hyphae grew, but needles and roots did not. The following spring from Mar to May 1995, sheath P content increased 1.2%/d. We suggest, on the basis of synchrony of changes of sheath P content with fungal and tree growth, that three seasonal phases of P dynamics in ectomycorrhizal sheaths can be distinguished: (i) uptake in early spring before tree growth, (ii) translocation to tree and fungus during spring, (iii) depletion during summer, fall, and winter when there was only fungal growth. Phosphorus in ectomycorrhizal sheaths may be shared by tree and fungus.