A sensitive gas chromatographic method has been developed for betanitropropionid acid (BNP), a 'compound produced by certain molds. The method is based on extraction, isolation and derivatization of the compound to yield its pentafluorobenzyl derivative and determination by electron affinity gas chromatography. The structure of the derivative was verified by mass spectrometry. The method has been applied to the determination of BNP in mold filtrates and cheeses with a limit of detection of about 1 and 3 ppm, respectively.