A method is described for the determination of fumonisin B1 in corn. The method involves sample extraction with methanol:water (75:25) and partial purification using a solid phase extraction. Fumonisin B1 is reacted with naphthalene-2,3-dicarboxaldehyde (NDA) to produce a highly fluorescent derivative, 1-cyano-2-alkyl-benz[f]isoindole (CBI) and then separated from the sample matrix on a reverse phase C-18 column with a mobile phase of acetonitrile:water:acetic acid (55:45:1). The NDA-derivative is quantitated by fluorescence detection at 410 nm excitation and a 440 nm, long past emission filter. Recoveries of fumonisin B1 added to corn at levels of 0.25–20.0 μg/g averaged 88.1% with a coefficient of variation of 10.3%. Confirmation of fumonisin B1 in corn samples was accomplished by fast atom bombardment (FAB) spectroscopy.