One hundred and six strains of A. fumigatus were isolated from 21 sawmills in Sweden, and 73 of these strains were examined for production of fumitremorgen B and verruculogen (tremorgenic mycotoxins) on YES-medium using thin layer chromatography (TLC). Twenty-three strains (32%) were tremorgen producers and 50 strains (68%) were non-producers. Tremorgenic mycotoxins were detected in conidia of seven A. fumigatus strains. The amount of toxin varied between 0.6-8.0 microgram/10(8) conidia (mean value 2.3 micrograms/10(8) conidia, equivalent with 0.18%). No production of the mycotoxin gliotoxin was detected in 6 strains of A. fumigatus. No tremorgens were detected during mould growth on wood substrates, in spite of the use of different wood species (Scots pine, Pinus sylvestris: Norway spruce, Picea abies and birch, Betula spp.), dried versus non-dried wood, bark (pine), leached wood, and wood after various sterilization methods.