Soil fumigation with methyl bromide resulted in the dominance among the fungi of Trichoderma species and Penicillium piscarium in both the soil and rhizosphere. The inoculum source was a pine needle mulch applied following fumigation. Application of a microbiologically different mulch resulted in a different fungal population which was still detectable after 4 months. The fungal population in the subsurface soil remained at a very low level throughout the study period of 7 months. The bacteria responded immediately and strongly to fumigation. Numbers in the subsurface soil subjected to high levels of methyl bromide exceeded the numbers in the controls 10 to 20 times immediately after removal of the fumigation covers. Their populations fell sharply but remained above the control populations throughout the study. The fungal species isolated from the rhizosphere were the same as those isolated from the soil, but 7 of the 19 species isolated from surface-sterilized roots were not recorded from either the soil or rhizosphere. Larger numbers of fungi in genera known to include plant pathogens were isolated from the surface-sterilized root tips of seedlings grown in nonfumigated soil than from fumigated soil. Clamp-bearing basidiomycetes were isolated from roots from all treatments. Histological and morphological examinations of roots indicated that fumigation delayed mycorrhiza formation for several weeks and altered the fungal symbionts involved.