Cultures of P megasperma var. sojae [soybean pathogen] grown on lima bean agar in petri dishes were flooded with distilled water to induce formation of zoospores. Maximum numbers of 104-105 zoospores/ml (15 ml of suspension per plate) were produced when 6 day old cultures of isolates of races 1 to 6 received 5 changes of distilled water each at 30 min intervals, followed by incubation in darkness at 20.degree. C for at least 8 h.