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Weedy Amaranthus species pose a serious threat to agriculture. One of the areas that causes the greatest concern is development of herbicide resistance and subsequent transfer of herbicide resistance genes among the species. To determine the potential impact of interspecific hybridization, one must first be able to detect such hybrids. To determine if genome size could be useful in the detection of interspecific hybrids among the weedy Amaranthus species, the genome sizes of the weedy Amaranthus species sympatric in Illinois were analyzed. In a series of experiments, the genome sizes of these species were determined by flow cytometric analysis. A significant variation was observed with respect to nuclear DNA content in the eight species examined. The genome sizes ranged from ≅ 0.95 pg in A. palmeri to ≅ 1.4 pg in A. tuberculatus. Overlap of genome sizes among the species does exist; however, due to the reproductive biology of the species, this overlap does not preclude the detection of interspecific hybrids. Any dioecious weedy Amaranthus plant in Illinois that has a genome size between 1.3 pg and 1.1 pg is probably a hybrid. Thus, the potential for genome size determination to reveal interspecific hybrids among weedy Amaranthus species has been demonstrated., WEEDY AMARANTHUS species (pigweeds) have been and continue to be a major problem in agronomic production. A major contributor to the noxious nature of these weedy species is their ability [...]