Technical Abstract: Corn may be colonized by the fungus, Fusarium verticillioides, resulting in both plant disease and mycotoxin contamination. The purpose of the current research was to compare frequencies of three F. verticillioides populations in kernels of corn grown under field conditions. The populations assessed were 1) an introduced transformant designated RRC PATg containing genes coding for hygromycin resistance and beta-glucuronidase (GUS), 2) naturally occurring genotypes, and 3) hygromycin resistant strains that did not have GUS expression. Population frequencies in kernels were quantified by placing surface sterilized kernels on media selective for each population combined with histochemical characterization of the mycelia. Kernels were examined from non-inoculated ears, as well as ears inoculated with either water or the genetic transformant, F. verticillioides PATg. The introduced population was not found in any non-inoculated ears, but was positive in approximately 75% of the kernels from F. verticillioides PATg-inoculated ears and <1% of kernels from water-inoculated ears. In contrast, at least 75% of the kernels were positive for naturally occurring F. verticillioides populations in non-inoculated ears and inoculated ears. In summary, the majority of kernels of both inoculated and non-inoculated ears were colonized by naturally occurring F. verticillioides, but only inoculated ears were colonized by the introduced F. verticillioides.