We developed a simple, rapid, small-scale assay for infection of tobacco seedlings by Phytophthora parasitica var. nicotianae. One 7-day-old tobacco seedling was placed in each well of a 96-well microtiter plate and inoculated with 500 zoospores of P. parasitica var. nicotianae. After 72 h all of the inoculated seedlings of the susceptible cultivar, KY14, were infected, and the pathogen had produced sporangia that were visible on the surfaces of the seedlings. Sporangia did not develop on seedlings that were inoculated simultaneously with zoospores and either 1-mu-g/mL of the chemical fungicide metalaxyl or 5-mu-L of filtrate of a sporulated culture of the biocontrol agent, Bacillus cereus UW85. Seedlings of tobacco cultivar KY17 were infected by P. parasitica var. nicotianae, although mature plants of this variety are resistant to the pathogen. This microassay may facilitate the rapid screening of potential biological and chemical control agents and may be useful for studying mechanisms of infection and control of Phytophthora spp. under hydroponic conditions.