Introgressive hybridization has important management and evolutionary consequences in regard to the maintenance of the integrity of gene pools and the acquisition of new alleles by means other than mutation. Among North American cypriniform fishes, hybridization often occurs between historically allopatric forms placed into sympatry via human activities and between sympatric forms inhabiting a modified environment. In the Colorado River system, several native catostomids are known to hybridize, including flannelmouth sucker Catostomus latipinnis and razorback sucker Xyrauchen texanus. Allozymic data enabled direct quantification of presumed introgression which was in the range of 0–5% toward flannelmouth sucker and 0–3% toward razorback sucker. Levels of presumably introgressed genes in the stock of razorback sucker from the Dexter Fish Hatchery were as low as or lower than those of the parental stock population from Lake Mohave.