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Progress Report: A comparison of morphology, thermal tolerances, and biochemical genetics of the Kendall Warm Springs dace (Rhinichthys osculus thermalis) and speckled dace Q. osculus yarrowi) of the upper Green River drainage in Wyoming. Calvin M. Kayal, Peter F. Brussardl, David G. Cameronl, William R. Gould2, and Ernest R. Vysel IBiology Department, Montana State University, Bozeman; 2Montana Cooperative Fisheries Research Unit, Biology Department, Montana State University, Bozeman Abstract Comparisons of morphology, thermal tolerance, and biochemical genetics between Kendall Warm Springs dace (Rhinichthys osculus thermalis), listed as an endangered fish by the U.S. Fish and Wildlife Service, and speckled dace (R_ o_ yarrowi) of the upper Green River drainage have demonstrated both similarities and differences. Tolerances for high temperatures (critical thermal maxima) do not differ significantly. Electrophoretic examination of 26 loci have shown no variants at any locus in either population, and both populations are fixed at all loci. However, five of eight restriction enzymes reveal polymorphic mitochondrial DNA (mtDNA) in R. o; yarrowi, and only one enzyme revealed a polymorphic mtDNA in R. oo. thermalis. The most important morphological difference measured is in pharyngeal tooth formulas, with 86% of R. o_ thermalis lacking pharyngeal teeth in at least one secondary row, compared with less than 1% of R. o. yarrowi. R. oo. yarrowi have been spawned, incubated, and are being reared at 18, 24, and 28 C to determine thermal lability of meristic features, and whether those at the highest temperature will remain differentiable from R_ o_ thermalis. Introduction The purpose of this study is to determine differences or similarities between the Kendall Warm Springs dace (Rhinichthys osculus thermalis) and speckled dace Q. osculus, also known as R. o_ yarrow!) of the upper Green River drainage through comparisons of several categories of characteristics: morphology and meristics, physiological thermal tolerances, and biochemical genetics. These comparisons are intended to evaluate the validity of the present subspecific designation of the Kendall Warm Springs dace, which is listed as an endangered fish by the U. S. Fish and Wildlife Service. These dace were first designated as a separate subspecies by Hubbs and Kuhne (1937), on the basis of morphological differences from other R. osculus and on their isolation in a small, thermal spring creek. Since it is possible that some of these morphological differences could be environmentally induced by elevated temperatures during early development of the young, as recognized by Hubbs and Kuhne (1937), the validity of the subspecific designation has remained in question. . 5