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, .' ABSTRACT ); l~J Bonytai 1, humpback and rountai 1 chubs from the Colorado River < system, and the Utah chub from the Bonneville Basin and upper Snake River drainages are used to demonstrate the potential of chromo- some study, or cytogenetics, in fisheries biology. Cytogenetic analysis yields results that are not affectec! by age, so it is, useful for larval and juvenile identification, as well as determina- tion of closely-related and variable species_ All Colorado River species are differentiable from each other on the basis of chromo- some states. Humpback chubs from Black Rocks, Colorado can be distinguished from conspecifics inhabiting the Little Colorado River, Arizona. Utah chubs have a chromosome morphology distin- guishing them from all Colorado River species. The potential uses of cytogenetics are discussed, to include genetic disease analysis, as well as the assessment of pollutants and radiation. Chromosomes can be used for sex determination in certain taxa or in ascertai~ ning genetic affinities. The development of suitable tissue culture techniques allows cytogenetic, as well as electrophoretic, analyses without the sacrifice of animals. I NT RODUCT ION ~ ~ Fishery biologists need procedures that allow discrimination among groups of fishes. Although external morphology, the familiar counts and mea- surements of Hubbs and Lagler (1964), is most often used to identify fishes, the methodology may fail with larvae and juveniles of related species or with the adults of closely-related variable ones. This identtfication problem has been successfully approached, in some cases, by the direct ex~mination of gene products wi th p rote i n elect rophores is (see A 11 endo rf and Utte r, 1978), where caution should be exercized with the choice of assay proteins (Sarich, 1977) or data interpretation (Thorpe, 1982). Another avenue for stock and species problems rema i ns totally unexp 10 i ted. / Eve ry organ i sm is characteri- zable by its chromosomes, or karyotype. Chromosomal features are fixed~' usually being sex, population or species specific, as well as age-independent (Swanson et al.', 1981). Although chromOsome methodology has had' losive deve lopment for tetrapod vertebrates,' such techniques have yet wi ly