Aristolochene is the likely precursor of the sesquiterpenoid toxins produced by a number of filamentous fungi. One of these, PR-toxin, is produced by Penicillium roqueforti. We report here the isolation of a gene (Ari1) coding for the sesquiterpene cyclase, aristolochene synthase (AS), from P. roqueforti. Nucleotide sequence analysis of genomic and cDNA clones revealed that the Ari1 gene contains two introns. A Protein A/AS fusion enzyme was expressed in Escherichia coli and shown to have sesquiterpene cyclase activity. Analysis of the Protein A/AS fusion enzyme reaction mixtures by TLC and gas chromatography/mass spectrometry identified aristolochene as a major product. The Ari1 gene encodes a polypeptide of molecular weight 39,200. Expression of Ari1 occurs in stationary phase cultures of P. roqueforti and appears to be transcriptionally regulated.