Zoospores of the fungus Phytophthora palmivora, pre-labeled with 45Ca, excreted up to 30% of their total 45Ca when stimulated to encyst. Excretion was essentially completed within 90 sec of the application of the stimulus. Encystment of the population was completed within 5 min. Four different stimuli were used: pectin addition (420 micrograms ml-1), Sr2+ addition (5 mM), cyclic AMP addition (6.7 mM) and mechanical agitation. The kinetics and amount of Ca excretion were essentially the same in each case. The calcium ionophore A23187 increased the rate of 45Ca uptake by motile zoospores, incubated in 100 microM CaCl2, but did not induce encystment under these conditions. The ionophore did not induce 45Ca efflux from pre-labeled zoospores. Incubation in EGTA and in K+ failed to induce either encystment or 45Ca excretion. We conclude that rapid excretion of a significant proportion of the zoospore calcium is linked to the early stage of stimulus-induced encystment, and that this comes from an intracellularly located, non-cytoplasmic source, such as the peripheral vesicles, but that changes in cellular Ca2+ are not necessarily the single controlling factor in the induction of encystment.