Penicillium decumbens is able to epoxidize cis-propenylphosphonic acid (cPA) to produce the antibiotic fosfomycin [FOM; also referred to as phosphonomycin and (-)-cis-1,2-epoxypropylphosphonic acid], a bioconversion of considerable commercial significance. We sought to improve the efficiency of the process by overexpression of the genes involved. A conventional approach of isolating the presumed epoxidase and its corresponding gene was not possible since cPA epoxidation could not be achieved with protein extracts. As an alternative approach, proteins induced by cPA were detected by two-dimensional gel electrophoresis. The observation that a 31-kDa protein (EpoA) was both cPA induced and overaccumulated in a strain which more efficiently converted cPA suggested that it might take part in the bioconversion. EpoA was purified, its amino acid sequence was partially determined, and the corresponding gene was isolated from cosmid and cDNA libraries with oligonucleotide probes. The DNA sequence for this gene (epoA) contained two introns and an open reading frame encoding a peptide of 277 amino acids having some similarity to oxygenases. When the gene was subcloned into P. decumbens, a fourfold increase in epoxidation activity was achieved. epoA-disruption mutants which were obtained by homologous recombination could not convert cPA to FOM. To investigate the regulation of the epoA promoter, the bialaphos resistance gene (bar, encoding phosphinothricin acetyltransferase) was used to replace the epoA-coding region. In P. decumbens, expression of the bar reporter gene was induced by cPA, FOM, and phosphorous acid but not by phosphoric acid.