Eighteen corn samples collected from Thailand were analysed for fumonisins and aflatoxins by high performance liquid chromatography. Fumonisins B1and B2were detected in 16 (89%) and 12 (67%) samples at levels ranging from 63 to 18800ng/g (mean, 1790ng/g) and from 50 to 1400 ng/g (mean, 251 ng/g), respectively.Fusarium moniliformeandF. proliferatumisolated from Thai corn showed the production of fumonisins in corn grit cultures. Aflatoxins B1and B2were found in 15 and 10 samples at mean levels of 72 and 15 ng/g, respectively. It was noted that as many as 72.2% (13/18) of samples were coincidentally contaminated with fumonisins and aflatoxins. Based on these results, the interaction of fumonisin‐producers and aflatoxin‐producers on kernel infection and contamination of these mycotoxins in corn is discussed. This is the first report on the natural co‐occurrence of fumonisins and aflatoxins in Thailand.