Chloroform production was detected in the headspace of pure cultures of the basidiomycetes Mycena metata and Peniophora pseudopini and the deuteromycete Caldariomyces fumago. The average production rates were in the range of 0.07-70 μg/l culture fluid/day for Caldariomyces fumago and 0.7-40 ng/1 culture fluid/day for the basidiomycetes; they depended on the composition of the medium, pH and the initial concentration of oxygen. In incidental cases, chloroform was identified in the headspace of pure cultures of the basidiomycetes Agaricus arvensis, Bjerkandera sp. BOS55, and Phellinus pini. It is suggested that fungi are important sources of elevated concentrations of chloroform in soil air.