Five fungal species, Suillus grevillei, Boletinus cavipes, Inocybe kobayasii, Lactarius porninsis and Gomphidius maculatus were examined for ergosterol content during mycorrhizal formation. Ergosterol assay was a suitable method for monitoring establishment of 5 ectomycorrhizal fungi on roots. Ergosterol contents were positively correlated to biomass of each of 5 ectomycorrhizal basidiomycetes growing on Melin-Norkrans medium, ranging 1.78-2.36 mu g mg(-1) dry mycelia. These fungi formed ectomycorrhizae with Japanese larch seedlings in container-grown axenic culture. There were significant correlations between overall mycorrhizal density and seedling growth such as root length and shoot height at 6 mo. Mycorrhizal seedlings accumulated more mass in the root system and mycorrhizae compared with nonmycorrhizal seedlings.