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natural populations of razorback sucker, and 2) variation within and among several year classes produced through a breeding program at DNFH. MtDNA VARIATION 1N NATURAL POPULATIONS Materials and Methods Source of samples.- Razorback suckers were once widely distributed, occupying large rivers and other floodplain habitats throughout the Colorado River basin, including the vast Gila River drainage of Arizona-New Mexico. Today, razorbacks are sparsely distributed, occurring in sizable numbers in only a few areas (Minckley et al., 1991). To examine the distribution of mtDNA variation, tissue samples were obtained from the following locations (Figure 1): Lake Mohave, Lake Mead, Lake Powell, Green-Yampa River, and upper Colorado River near Grand Junction (Table 1, Appendix 17. Tissue samples were also obtained from razorbacks (or putative hybrids with flannelmouth suckers, Carostomus latipinnus) from the Colorado River Indian Tribal (CRI1~ canals in southwestern Arizona and the Little Colorado River. The upper Colorado River sample is noteworthy as it consisted mostly of tissues from ` individuals collected from two floodplain ponds; only two individuals were obtained from the Colorado River proper. Since these ponds are isolated from the river except during high flood, it is possible that each individual does not provide an independent sample, as they may have been derived from one or a few females. Examination of otoliths indicated all individuals belong to the same year class (Minckley, unpublished data), and allozymes (Buth and Haglund, unpublished data) identified many of the individuals as products of hybridization between razorback and flannelmouth suckers. Both of these observations are consistent with the hypothesis that individuals from these ponds are closely related and derived from a small number of female parents. Therefore, the upper Colorado liver sample 3