Zoospores of P. parasitica were present in water placed on field soils from foot-rot infested citrus groves within 10 min, and citrus leaves in the water were infected within the same time. Zoospores retained motility in irrigation water up to 20 h at 20.degree. C, but a higher temperatures the period of motility was shorter. Zoospores encysted rapidly after the addition of nutrients, orange peel or citrus leaves. Zoospore cysts germinated in sterile distilled water (SDW) and sterile and nonsterile irrigation waste water. When growth ceased, the protoplasm contracted within the hyphae. Empty cysts or hyphae often lysed, but intact hyphal fragments remained viable for 40 days at 25.degree. C in untreated waste water and resumed growth after the addition of supplemental nutrients. At concentrations of 10-1000 mg glucose/l of SDW plus 100 mg CaCl2 or in the presence of orange peel or citrus leaves, hyphal tips in contact with container surfaces often produced appressoriumlike structures which usually produced microsporangia when nutrients were replaced with autoclaved or nonsterile irrigation waste water. Microsporangia could persist in nonsterile irrigation waste water at 24.degree. C for 60 days, but most germinated sooner, releasing only 1 zoospore. Since mycelial inoculum from these zoospores was pathogenic to roots of citrus seedlings, zoospores of P. parasitica or structures produced by them probably play a significant role as survival or dispersal units in recycled water.