Seventy-one strains of fungi belonging to 53 species of Zygomycota and mitosporic fungi were isolated from soils containing different levels of 2,4,6-trinitrotoluene (TNT). Three moderately contaminated samples (500 ppm) and an uncontaminated control contained a rich diversity of species whereas few fungi were isolated from a heavily contaminated sample (1000 ppm). Most isolates were capable of biotransforming TNT into aminodinitrotoluenes and azoxy-dimers, although they differed widely in their tolerance of TNT. Spore germination was generally more strongly inhibited by TNT than hyphal growth. Among the Zygomycota, Absidia and Cunninghamella were highly TNT-tolerant and attacked the substrate even when this had precipitated at 1000 ppm, whereas 12 of the 14 Mortierella isolates were intolerant of TNT above 50 ppm. Among the mitosporic fungi, Acremonium, Cylindrocarpon, Gliocladium and Trichoderma but not Penicillium species were highly tolerant of TNT and had a high capacity to biotransform this compound. Large variations in these properties were determined between species and even strains belonging to the same species, and between different isolates from the same soil sample. Populations of soil microfungi therefore show a high natural variation of TNT tolerance and biotransformation ability, irrespective of any previous long-term exposure to this xenobiotic.