Summary Results are presented from several studies on the epidemiology of Armillaria ostoyae in Douglas-fir plantations in the interior cedar-hemlock (ICH) biogeoclimatic zone of British Columbia. Two plantations were monitored for mortality by A. ostoyae and other agents for 35 years after establishment. In these and other plantations ranging in age from 7 to 32 years, one or more of the following factors were determined: source of inoculum, mode of spread and characteristics of lesions on roots of excavated trees; symptom expression in relation to tree age and damage to the root system and years from initial infection to death on trees killed by the fungus. Mortality from A. ostoyae began in both plantations about 5 years after planting, reaching 30% in one and 11% in the other after 35 years. The spatial pattern of mortality was similar to that reported from New Zealand, France and South Africa; however, the temporal pattern differed, beginning later and, instead of declining, continuing at a nearly constant rate to the present. To age 10, nearly all infections were initiated by rhizomorphs; as plantations aged, the proportion of infections occurring at root–root contacts increased. In seven plantations, in moist and wet subzones of the ICH, from 23 to 52% of Douglas-firs had root lesions, with the higher incidences occurring on moist sites. The occurrence of aboveground symptoms, reduced leader growth and basal resinosis, was related to the percentage of root length colonized by A. ostoyae on trees with more than 30% of root length killed. Average time from infection to death increased from 1 to 2 years at age 6 to 22 years at age 33. The outlook for timber yield from Douglas-fir plantations in the cedar-hemlock zone is discussed. Management alternatives for reducing damage from A. ostoyae when regenerating sites are reviewed.