Populations of Pythium and Mortierella species were estimated by plating surface soils in agar media containing pentachloronitrobenzene, pimaricin, and bacterial antibiotics. Pythium spp. were found in 16 of 17 Ontario soils, mainly seedbed soils in forest nurseries. There was significant variation in the mean population density of Pythium propagules between two seedbeds sampled in each nursery and between the nurseries. The population was high in 10 soils (7 to 57 propagules per cm3) with P. irregulare as the dominant species. The following species were fairly abundant (in decreasing order): P. rostratum, P. ultimum, and P. oligandrum. Pythium was not significantly correlated with properties of the nurseries or with populations of Mortierella, bacteria, Streptomyces, and heat-tolerant fungi. The Pythium population was high only in old nurseries. Mortierella spp. occurred in all soils varying from 1 to 500 propagules per cm3.