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Objectives were addressed as a sequence of tasks, each to be completed before the next could be addressed. 1. Obtain tissues of the three study species and their putative hybrids from the study area. 2. Extract genomic DNA from samples using a commercially available kit. 3. Screen several introns for levels of suitable genetic diversity -that is, determine wihin each intron regions that are not only species-specific and unambiguous, but which reflect no within-species variation. This required that the introns must be evaluated in samples taken from across the Colorado River Basin. 4. Develop an efficient methodology that would allow >500 samples to be processed. 5. Evaluate samples using these primers and allocate individuals to one of six genotypes: FMS, BHS, WHS, FxW, BxW, or FxB. 6. Compare genotypes to phenotypic designation of individual at time of capture (the latter based on coloration, scalation, body shape, and lip morphology). 7. Determine spatial distribution of the genotypes by allocating them to study reach. 8. Es#imate age of hybrids by evaluating genotypes according to total length (a surrogate for age). Results demonstrated that morphological and genotypic identifications were congruent in over 95% of the individuals. First generation (i.e., F1) hybrids are usually morphologically intermediate between parental species, whereas backcrossed individuals may tend to resemble one of the two parentals. The majority of hybrid individuals in the Yampa River seemingly represented F1s, in that genetic determination of hybridity confirmed morphological diagnosis. However, two hybrids between Flannelmouth and Bluehead suckers were found, and this suggests backcrossing as a possible scenario. These two species are found sympatrically throughout the Colorado River basin, but little or no hybridization has been documented between them to date. Introduction (invasion) of the non-indigenous White Sucker not only has created the poten#ial for hybridization between introduced and indigenous species, but also appears to have potentially compromised the boundary between the two indigenous species. 3