SUMMARYOne hundred and twenty-one isolates ofPythium ultimumTrow from diseased guayule (Parthenium argentatumGray) seedlings were classified into 3 types on the basis of relative abundance of oospores and sporangia (chlamydospores) when grown on Difco cornmeal agar. Three of the isolates which produced mainly oospores were classified as type O; 71 which produced oospores and few or many sporangia were classified as type OS; and 47 which produced only sporangia were classified as type S.Twenty hyphal-tip subcultures were taken from each of 25 selected isolates. The subcultures from three O type, 11 OS type and 7 S type were similar to the parent cultures; those from 4 OS type isolates segregated into two that gave type O and type S subcultures and 2 that gave type OS and type S indicating that these four isolates were mixed cultures or were unstable as to type.In an effort to induce isolates of the S type (non-oospore formers) to produce oospores they were grown on cornmeal agar, water agar, cornmeal agar plus an extract of guayule shrub, cornmeal agar plus an extract from well-decayed leaf mold, cornmeal agar plus soil extract, and agar made from freshly grated carrots. None of these media stimulated the formation of oospores in the S type isolates.Five O type, 13 OS type and 11 S type hyphal-tip subcultures of 25 isolates were found to have very similar rates of growth on cornmeal agar at each of the following constant temperatures: 10°, 15°, 20°, 25°, 30°, 35°and 40°C. The growth rate at these temperatures was comparable to that reported (8) forPythium ultimum.The optimum temperature for mycelial growth was found to be between 25°and 30°C.Twenty-six hyphal-tip subcultures which included four O type, 13 OS type and 9 S type were tested for pathogenicity on emerging guayule seedlings in the greenhouse. All of the different types were found to be pathogenic and of comparable virulence in causing preemergence loss to guayule seedlings.In general most hyphal-tip or single-spore subcultures ofPythium ultimumisolates were stable as to type. However, one isolate, 980, readily gave rise to other types. Successive hyphal-tip transfers of a hyphal-tip subculture of 980 of the OS type gave rise to both the O and S types as well as the parent type. Three generations of single-spore (sporangium) subcultures from two isolates of the OS type were all OS type similar to the parent. Likewise one single-spore generation of two S type isolates were all of the S type.