Patterns of calcium accumulation have been compared between two strains of the ectomycorrhizal fungus Paxillus involutes (Batsch. ex Fr.) Fr. Calcium accumulation during a period of growth on a substrate containing calcium paralleled that found in higher plants, with the calcifuge strain accumulating higher levels of calcium than the calcicole strain, especially in the older parts of the mycelium. In short-term uptake experiments, using mycelia grown on calcium free medium then placed for 3 h in various calcium solutions, this order was usually reversed with the calcicole strain accumulating higher levels than the calcifuge strain. Accumulation in relation to calcium concentration was biphasic, and was generally higher at 4 °C than at 24 °C. This is inconsistent with an equilibrium model based on passive mechanisms but is consistent with a model which has active calcium efflux balancing a large passive influx. The operation of this mechanism in mycorrhizal fungi could serve to protect calcifuge plants in calcareous situations.