Phytophthora spp. can infect plants within 30 min of a zoospore arriving at a host surface. The infection sequence from zoospore encystment to cyst germination and host penetration seems to be largely preprogrammed and dependent on calcium signaling. Here we present evidence that the encystment process leading to autonomous germination in Phytophthora parasitica may be coordinated by sequential net Ca2+ fluxes across the cell membrane, which we assessed by fluorimetry of the probe fura-2 in zoospore bathing medium. Encystment was associated with a large initial net Ca2+ influx, which was abolished by the channel blockers La3+ and verapamil. The net influx was followed by a larger, progressive net Ca2+ efflux over 20-30 min, which was associated with cyst germination but was inhibited by TMB-8, implicating Ca2+ release from intracellular stores. The effects of inhibitors were overcome by high (1 mM) exogenous Ca2+, enabling encystment and germination. Copyright 1998 Academic Press.