A method for determining fumonisin B1(FB1) in corn was developed and the clean-up optimized in order to give an extract suitable for one-dimensional thin layer chromatographic (TLC) analysis. FB1was extracted with a solution of methanol:water (80:20,v/v), purified through an immunoaffinity column and separated on a C18reversed phase TLC plate. The FB1was visualized with 0.1mol/l sodium tetraborate, 0.40mg/ml fluorescamine in acetonitrile and 0.01mol/l boric acid:acetonitrile (2:3,v/v) for fluorescence detection, and quantified by densitometric analysis. Water, acetonitrile:water (1:1v/v) and acetonitrile:water (4:1v/v) were evaluated as TLC solvents for running both standards and samples together with derivatization procedures aimed at improving separation, resolution, sensitivity and linearity. The mean recovery for FB1for spiked samples was found to be 85% and the linear equation of standard calibration curve by densitometric analysis gave an r2value higher than 0.99. The maximum coefficient of variation for replicate analysis of spiked samples was 19%. The absolute amount of FB1standard detectable on a TLC plate was 2 ng, giving a detection limit for the method of 0.1mg/kg. The method has been shown to be robust in the application of FB1monitoring in corn (214 samples) collected in different regions of the country. FB1was detected in 99% of these samples in the range of 0.2 to 6 mg/kg.