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Incorporations of l3C-labeIled acetates and methionine into the mycotoxin austin in cultures of Aspergillus ustus give a labelling pattern consistent with a mixed polyketide-terpenoid pathway. Incorporations of 14C and *H labelled 3,5-dimethylorsellinate confirm the intermediacy of a preformed tetraketide-derived phenolic precursor. Further information on the mechanisms involved in the modifications of both the farnesyl-and tetraketide-derived portions of the molecule are provided by i n c o r po ra t i o n stud i es with [ 1 -3C, 0 ,I acetate, [methyl-C ,* H 3] met h ion i ne, 3C, 8O -I a be I I ed d i m et h y Iorsellinate, I8O2 gas and [6-13C,6-*H3] mevalonic acid lactone. Aspergillus 11stii.5, one of the most prevalent fungi in soil and decaying vegetation, is a known contaminant of stored foodstuffs such as cereals, pulses and cheese and is known to produce a number of toxic secondary metabolites of varied biosynthetic origin.The first secondary metabolite isolated from A . ustus in 1951

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