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Geographic isolation would result in grouping of haplotypes (i.e., monophyly) from the same region whereas considerable gene exchange among regions would be reflected by limited correspondence between geographic and phylogenetic partitioning of haplotypes in networks. Results Restriction endonuclease analysis of mtDNA from 146 specimens sampled with eight enzymes produced approximately 160 fragments per individual, yielding 38 composite haplotypes ('Table 2, Appendix I). Infom~ation was also obtained from an additional 15 individuals; seven individuals were only partially analyzed due to poor quality of DNA, and eight specimens were found to exhibit C. latipinnis mtDNA. In all cases, changes in razorback fragment patterns resulted from restriction site gains or losses, with no evident variation due to length differences. Hybridization.- Given the general lack of recruitment of juvenile razorbacks, there have been concerns over hybridization between razorbacks and other suckers, especially flannelmouths (Hinckley et a]., 1991). Buth et al. (1987) found no direct evidence of hybridization in razorbacks from Lake Mohave. Analysis of mtDNA revealed essentially the same, as only one of 119 fish morphologically identified as razorbacks from lakes Mohave, Mead, and Powell or the Green/Yampa rivers exhibited flannelmouth mtDNA. _ Introgression appeared to be more significant in two other sets of samples. In t'le upper Colorado River, two of 29 specimens exhibited flannelmouth mtDNA; however, as noted above, most (including the two in question) were collected from ponds isolated from the mainstem. In such ponds, a small number of razorbacks may have been isolated with other suckers, increasing the probability of hybridization. In a Little Colorado River sample, five of 12 individuals (all morphologically identified as hybrids due to intermediate color~patterns 7