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citing early 1960s agency reports) reported that Razorback Sucker was rare in the Green and Yampa rivers. Following closure of Flaming Gorge Dam, Vanicek et al. (1970) then collected 16 putative hybrids from the Green River. Vanicek (1967:45) also noted that morphological characters of hybrids were intermedia#e between those of the two parental species. This was in agreement with Hubbs and Miller (1953), who recorded similar findings from an examination of eight total specimens, two from the upper Colorado River and six from the upper Green River. Holden (1973) reported that hybrids were collected throughout the range of the species, usually in quiet, backwater areas in assoaation with the parental species. Holden also noted that Razorback Sucker were collected only in the middle and lower sections of the upper Colorado River basin, and were considered "rare." In his study, Holden collected 40 hybrids and 53 razorbacks. He noted that hybrids were readily distinguished by an intermediate lateral line scale count and by an abbreviated (although clear) keel. Buth et al. (1987) used allozymes to examine potential for natural hybridization in C. latipinnis and X. texanus collected throughout the Colorado River basin. Overall, the propensity was 0--3% toward X. texanus and 0--5% toward C. latipinnis. Of 41 putative hybrids morphologically identified by Douglas and Marsh (1998) in the Little Colorado River of Grand Canyon, nine (22%) were examined electrophoreticallysnd with RFLP analysis of mtDNA. Eight of these proved to be backcrossed. This suggests that morphological identification of putative hybrids was reasonable, in that 89% (8/9) were correctly identified. It also indicated that a very small subpopulation of hybrids exists within the Marble Canyon reach of Grand Ganyon. It averages 30 in number, is predominantly male, and frequents the Little Colorado River in spring, on average 2.2 km above the confluence. Lastly, results from Douglas and Marsh (199$) suggest that hybrids are indeed fertile and capable of backcrossing with parental forms. The present study also focuses upon introgressive hybridization among suckers (Catostomidae), but specifically between introduced White Sucker (Catostomus commerson~) and the endemic Colorado River Basin Flannelmouth Sucker (C. latipinnis) and Bluehead Sucker [C. (Pantosteus) discobolus]. These three species 7