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between the upper Rio Grande proper and Pecos samples occur at the EST-2 and GPI-1 loci (Figs. 6,7) but the extremely small Pecos sample renders this highly inconclusive. DeMarais (1992) did not examine EST-2 but found two alleles in samples from the two subbasins at "GPI-A" which occurred at similar frequencies throughout; thus differences at GPI-1 herein, which had but a single allele in the Pecos sample, might diminish with increased sample size and/or the addition of samples from other populations within these subbasins. Taxonomic Implications Gila forms from the Guzman and Rio Grande-Pecos basins have been variously considered to represent a single taxon, G. nigrescens (Girard, 1856)(e.g., Koster, 1957; Bailey et al., 1970) or populations from the Rio Grande-Pecos have been considered a separate taxon, G. pandora (Cope, 1872) as suspected by Miller (1945)(e.g., Smith and Miller, 1986; Sublette et al., 1990; Robins et al., 1991; DeMarais, 1992; Starnes, in press). Allelic differences noted above, mainly supplemented by the findings of DeMarais (1992), provide further justification for recognition of both taxa. Moreover, possible differences among populations within those basins bear further investigation, particularly with regard to the EST-2 locus. It is particularly noteworthy that there were no fixed differences between the Guzman Basin sample (G. nigrescens) and samples from the Gila Basin at any of the loci examined whereas, if differences at the GPI-1 and PK-1 loci withstand further scrutiny, such a distinction may exist between the Guzman and Rio Grande basin (G. pandora) populations. Colorado River Basin Genotypic variation within Colorado Basin Gila has regional characteristics. Discussion of these characteristics is therefore compartmentalized on a regional basis preceded by an overview on inter- and intrabasin variation. For purposes of discussion the Colorado Basin is divided into three regions, following, in part, the convention of Minckley et al. (1986). The Lower Basin consists of the Gila and Bill Williams river systems and lower mainstem of the Colorado River below the Bill Williams confluence (Fig. 1). The Middle Basin includes the mainstem Colorado from Lake Mohave upstream to Glen Canyon Dam near the Axizona-Utah border, and tributaries of that region, the Virgin, Moapa, and Pluvial White systems and the Little Colorado River. The Upper Basin comprises the Colorado River and all tributaries, including the San Juan and Green river systems, from Lake Powell upstream. Taken collectively, samples of Gila taxa from the Colorado Basin exhibit genotypic distinctness from those taken from basins 8