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1 1 1 1 1 1 1 1 1 11 Allozyme variability in juveniles.- Allele frequencies at polymorphic loci in samples of juvenile Colorado pikeminnow from the Colorado, Green, and Yampa rivers are summarized in Table 4. Hypotheses that would presume spawning aggregates to be reassembled at natal areas would likewise suggest that juveniles might have moved throughout the drainage, thereby homogenizing allele frequencies. Though sample sizes were smaller than for adults and no San Juan River juveniles were sampled, seven loci were polymorphic and 17 of 22 identified alleles were present in the juvenile sample. PGAM-2 deviated significantly from Hardy-Weinberg expectations in Cataract Canyon juveniles (N=7) from below the confluence of the Green and Colorado rivers. While this deviation may indicate historic genetic differentiation, the low mean FS, (0.049) among juvenile samples suggests that sampling error is a more likely cause. Allozyme variability in Young-of-the- Fear.- Allele frequencies at isozyme loci in YOY Colorado pikeminnow are presented in Table 5; similar to the juvenile data set, no YOY were collected in the San Juan River for this study. The sampling protocol was designed to collect YOY during the fall, shortly after spawning below putative spawning sites, thus both the adult and the YOY samples were presumed to reflect accurately the genetic composition of spawning populations. Because the YOY samples were much larger than either adult or juvenile samples, it could be expected that detection of genetic- differentiation should be most probable in the YOY samples. Also, YOY populations were captured during two years (see Appendix 1; Table 3). As expected, many rare alleles were observed in YOY samples, including most of the few alleles restricted to single river systems, GPI-2 *c in the Colorado and GR*b and TPI-2 *a in the Green River system. The TPI-I*A allele found by Ammerman and Morizot (1989) only in Green River YOY was not observed in the present study. Seven loci were polymorphic in the YOY samples, with only the *b allele at PEPS b and TPI--1 *a being absent; the YOY fish from the Colorado River exhibited 19 of 22 identified alleles, slightly more than the 18 of 22 alleles observed in Green River YOY. Four loci (GPI-2 and PEPB in lower Colorado River samples and PEPB and PGAM-2 in lower Green River samples) deviated significantly from Hardy-Weinberg expectations; such deviations also were observed by Ammerman and Morizot (1989). While localization of non-equilibrium genotypes in lower reaches is strongly suggestive of admixture of differentiated populations and/or assortative mating, the mean F. among YOY population samples of 0.046 is consistent with minimal genetic differentiation at present (Wright 1978) Genetic variability in hatcherypopulations.- Five captive populations maintained or produced at the Dexter NFH&TC have been assessed for allozymic variability, three (Yampa River DX-Fi(74), Colorado-Green River DX-F1(81), and Yampa River F2(91)) in the present study and two (Colorado-Green River DX-FI(81) and Yampa River DX-F2(87)) by Ammerman and Morizot (1989). Seven of nine informative loci were polymorphic in at least one hatchery population, with 18 of 22 alleles represented (Table 6). While no loci deviated significantly from Hardy-Weinberg equilibrium, frequencies of several alleles were highly variable. In particular, the 0.33 frequency of the GPI-2*a allele in the Dexter 1974 sample was three times higher than any wild population sample. The Dexter 1974 hatchery broodfish population was derived from 8 or fewer adults collected in the Yampa River in 1973: the high GPI-2 *a frequency thus could result from higher frequency of this allele in past decades or from over representation of an uncommon allele in a few founders. In spite of potential founder effects, hatchery samples cluster with wild adult populations with genetic similarities >0:985 (Figure 2). Genetic identification using nuclear DNA.-- No genetic polymorphisms were detected in individuals from four populations of Colorado pikeminnow using gastrin-like PCR product digested with 10 restriction enzymes (Appendix 2; Table 1). Similarly, no polymorphisms were detected using thyroxin- like PCR product digestions with 10 restriction enzymes (Appendix 2; Table 2). Examples of lack of polymorphisms are presented in Appendix 2 (Figures 1 and 2). DNA sequences associated with the thyroxin and gastrin loci in Colorado pikeminnow appear to be highly conserved and thus invariant. The lack of variability made the examination for population differences moot.