SUMMARYThe endogenous Qo2of mycelium ofPythium aphanidermatumvaried from 35 to 2. Young mycelium had the highest endogenous Qo2values. Starvation of mycelia of different ages reduced the Qo2value to an apparent endpoint value of 2 after 5 days of starvation. Maximum Qo2values, approximating 35, were reached in mycelia grown for short time periods in water extracts of lima beans. The Qo2of young mycelium grown in chemically defined liquid media containing only essential salts and sucrose and N03-N as carbon and nitrogen sources approximated 13. The addition of glucose or sucrose to mycelium held under starvation conditions for 5 days resulted in a 2–3-fold increase in oxygen consumption after 1 hr. Young mycelium with high endogenous respiration rates had the greatest capacity for sporangium and zoospore production.