Concentrations of phosphorus (P), potassium (K), magnesium (Mg) and calcium (Ca) were determined in situ in fully hydrated arbuscular mycorrhizas by cryoanalytical scanning electron microscopy. The field- and glasshouse-grown plants (subterranean and white clovers, field pea and leek) were colonized by indigenous mycorrhizal fungi.The [P] in intraradical hyphae was generally 60-170 mM, although up to 600 mM was recorded, and formed strong linear relationships with [K], up to 350 mM, and [Mg], up to 175 mM. Little Ca was detected. The turgid branches of young arbuscules contained 30-50 mM P, up to 100 mM K and little Mg. Collapsing arbuscule branches and clumped arbuscules had greatly elevated Ca (30-250 mM), but otherwise differed little from young arbuscule branches in elemental concentration.The [P] was low or undetectable in 86% of uncolonized cortical cell vacuoles, but was generally elevated in vacuoles surrounding an arbuscule and in the liquid surrounding hyphae in intercellular spaces.Our results suggest that both young arbuscules and intercellular hyphae are sites for P-transfer, that Mg2+ and K+ are probably balancing cations for P anions in hyphae, and that host cells may limit arbuscule lifespan through deposition of material rich in Ca.