The rate of the ⁴⁵Ca²⁺ uptake by the submerged Trichoderma viride mycelium increased with the age of the culture from 6 h until a maximum which was reached at about 30 h, and then decreased until the uptake was virtually zero. The decrease in the rate of the ⁴⁵Ca²⁺ uptake was accompanied by an increase of mycelial mass. The uptake rate could not be reactivated upon substituting the medium for a fresh one, without or with dilution of the mycelium. The results suggest that the rate of ⁴⁵Ca²⁺ uptake reflects the biological age of the submerged culture. The surface-cultivated mycelium took up ⁴⁵Ca²⁺ proportionally with time. The autoradiography of colonies showed that ⁴⁵Ca²⁺ was distributed homogeneously throughout the mycelium during vegetative growth while conidiation was accompanied by a massive accumulation of ⁴⁵Ca²⁺ in conidia.