AbstractOut of 140 soybean samples tested from 17 countries, 42 originating from Brazil, Colombia, Iran, Korea, Malaysia, Thailand and the United States were found to be contaminated by oospores ofPeronospora manshurica(Naum.) Syd. Oospores are generally present as a crust on the seed coat, and are easily detected during visual examination; however, individual spores may stick to the surface of non‐crusted seed, and these spores are detectable only by examining surface washings.A new method for testing the viability of oospores by the tetrazolium chloride test is described. Viable oospores turn orange‐red. In all oospore collections, viability tested by this method was generally higher than germination recorded in water. One to 2‐year‐old oospores showed 30 to 39% viability, while an 8‐year‐old collection had about 20% viable spores. Oospores collected from 1‐year‐old seed treated with captan showed 6 to