The epizootiology of Entomophthora canarjensis Macleod, Tyrrell, & Soper in populations of the woolly pine needle aphid (Schizolachnus piniradiatae (Davidson)) was investigated during a 5-year period in an isolated plantation of red pine (Pinus resinosa Aitj in northern Ontario. Climatic factors did not limit the spread of the fungus; the major controlling factors were the density of the fungus inoculum and the density and spatial distribution of ti1tl host. An epizootic killed nearly 100 percent of the aphids in early October 1964, and in the subsequent 2 years E. canadensis began to spread at lower population densities and continued to spread faster than during 1961-64, probably because the inoculum density was greater after the epizootic.