Fungal spoilage of animal feed silage occurs frequently. In spoiled silage of sugarbeet press pulp sampled in The Netherlands during the period 1986–90, 40% of the samples were infested by Penicillium roquefortii. Other fungi of health significance included Aspergillus fumigatus (8% of all samples) and Byssochlamys spp. (4 % of all samples). P. roquefortii is also the dominant spoilage mould in maize silage. However, no PR-toxin could be detected in 25 lumps of P. roquefortii-intested maize silage, although one lump contained a fluorescent substance, with an Rff-value close to that of PR-toxin. This silage sample was not mutagenic, but had a cytotoxic effect towards Salmonella typhimurium in the Ames test. All P. roquefortii-infested lumps contained fluorescent fungal metabolites which were absent in samples taken at 5 cm distance from the corresponding lumps in the silage heaps. It is recommended that lumps of fungal-infested silage are removed before feeding the silage to cattle.