Oospores of two single zoospore isolates of P. fragariae from inoculated roots of F. vesca VS1 were extracted and placed on agar under various conditions. Less than 15% of one isolate germinated and germination was unaffected by age, while 40–70 % of young oospores less than four weeks old of the second isolate germinated. Germination of the second isolate's spores fell with age, the fall being accelerated if the roots containing the spores had been held at temperatures above 15 °C. Prolonged exposure to 30° in roots or on agar caused loss of oospore viability. Germination was optimum at 15°. Although slower at 10° and 3° and absent at — 1° it was rapidly recovered when spores from these temperatures were transferred to 15°. Temperatures above 15° progressively reduced the numbers of germinated spores and although more germinated when transferred to 15° the increase in their numbers was much slower than for spores held at lower temperatures. Low water potential created by adding NaCl, KCl, mannitol and sucrose to agar slowed germination but had little effect on the final numbers of active spores although germ tube formation was suppressed at water potentials of — 1.5 MPa. Spores germinated between pH 4–8 and the optimum for germ tube formation was pH 5.5-7.