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obvious heteromorphy involvingchromd;~me pai some,:pairs were assigned a number'according to theirre)ativelengths, the, largest being chromosome r;<;:They' are"considered as a single unit (e. g. chromosome' 1); since each member of a ,chromosome pair is ",equal in all para-' meters to the other one. Within bony tail , humpback and Utah chubs, chromo- some 19 has a centromere placed in such a manner that i,tchas a short upper arm and a longer upper one. In contrast, the upper and lower arms of this chromo- some in the roundtail chub is about equally divided by the centromere. Chromo- some 17 is unique for the Utah chub, relative to the other species. In the former fish, there is an extremely small short arm especially when compared to the same chromosome obtained from other congeners. This chromosome is also largest in the Utah chub relative to the Colorado River species. NOR methodology was only applied on chromosome spreads obtained from the native Colorado River species. The results do not indicate any differences between the round ta i 1 and bonyta n chubs but do sepa rate the humpback chub from the others (Fig. 2). An interstitial NOR on the long arm of chromosome 7 Is present in humpback chubs derived from Black Rocks, Colorado, but not in con- specifics originating from the little Colorado River. DISCUSSION Juvenile fishes were a component of this study and, as pointed out earlier, they are difficult to tell apart using external morphology. The cytogenetic results indicate that roundtail chubs are differentiable from bony tail and humpback chubs on the basis of a single chromosome. Further differentiatlon is achieved with the higher resolution NOR technique, which separates not only the bony tail chub from the humpback chub but also the Black Rocks and Little Colorado populations of the latter. Since chromosome char- acters are stable, these findings may have diagnostic value in future studies. If hybrids between any of the examined stocks were to occur in nature, they would be perceivable by cytogenetic analysis. Introgression involving a round- tail chub would yield individuals with a heteromorphic chromosome pair 19, one with a shorter upper arm than the other. If a Utah chub were to be a parent in a hybrid cross, then chromosome pair 17 would include one chromosome with an extremely-reduced lesser arm. Crosses involving humpback chubs are identi- fyable with NOR methodology. Verification of these predictions is in progress. Tissue culture is now being used in this laboratory to obtain chromosomes. The technique does not require that fish be sacrificed. Instead, analyzable chromosomes are obtained from fin clips which can be taken in the field'and stored on ice in an appropriate nutrient solution for up to two weeks. Fishes can be returned to the environment minutes after capture. Tissue culture offers another atvantage, since cells may also be used for electrophoretic characteri- zation. Consequently, a reasonably complete genetic profile may be established on animals. that must remain in nature, such as endangered species, or are to be kept al ive for. hatchery projects. . <. ::;:.';:-, ~r ._ 'v::"- _.:.:"