Commercially available ELISA tests for detecting Phytophthora and Pythium species were used to detect propagules of these genera in water samples. Samples were filtered through 0.45-mu-m filters to concentrate propagules before extraction. Heating filter residues to 100 C for 5 min yielded extracts that were equal or superior in reactivity to those obtained by liquid nitrogen disruption. As few as 30-40 zoospore cysts were detected in filter pad extracts. Detection sensitivity (propagules per liter) depends on the volume of water that can be passed through the filters. A water sample collected in late winter from a recycling pond in a northern California nursery did not contain any viable propagules of pythiaceous fungi or detectable antigen. A similar water sample collected in early spring from a southern California nursery contained 442 viable propagules per liter of water. The species recovered on agar media were Phytophthora parasitica, P. citrophthora, P. cryptogea, and an unidentified Phytophthora species, in addition to Pythium coloratum, P. rostratum, P. middletonii, P. ultimum var. sporangiferum, and Pythium 'L' group. Reactivity of filter extracts in Phytophthora ELISA tests was proportional to the quantity of filter residue extracted. All of the Phytophthora species recovered reacted positively with Phytophthora kits and negatively with Pythium kits. All of the Pythium species recovered also reacted positively with Phytophthora and negatively with Pythium kits. With a relatively simple filter-extraction procedure, the ELISA kits effectively detected Phytophthora and Pythium in water samples. Although the test are fallible at the genus level, they still could be used in commercial nurseries, where there is no known acceptable tolerance for these genera in irrigation water.