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INTRODUCTION The problem of hybridization between razorback suckers (Xyrauchen texanus) and flannelmounth suckers (Catostomus latipinnis) can be addressed using allozyme characters if suitable diagnostic "marker loci" are identified. These loci, that express different alleles in each of these species, would be expected to be heterozygous for the two parental alleles at all marker loci in the F1 generation. Reassortment in the F2 and subsequent generations, and backcrosses, would yield lesser degrees of heterozygosity. However, remnant alleles from the original hybridization event serve to indicate that such genetic contamination had occurred. Buth et al. (1987) found a surprisingly high level of genetic similarity between razorback and flannelmouth suckers (Neils 1978 genetic identity of 0.80) that would not have been expected of catostomid species that are not particularly closely related (following Smith 1992). Nevertheless, four marker loci were identified: creatine kinase (Ck-A), glucose-6-phosphate isomerase (Gpi-A2), isocitrate dehydrogenase (mIcdh-A), and superoxide dismutase (sSod-A2). No F1 hybrids were identified and the introgression at the four marker loci was unequal: low levels for Ck-A and mIcdh-A, no introgression at Gpi-A2 or sSod-A2. Our ongoing study of population structure in razorbacks is limited to non-lethally sampled tissues (muscle and fin), which means that Ck-A and mIcdh-A (both from muscle) could be studied but that Gpi-A2 (expressed in brain) and sSod-A2 (expressed in liver) could not be studied. The first part of our study included a rigorous examination of enzyme expression that employed many buffer systems. This effort identified two more marker loci: mitochondrial malate dehydrogenase (mMdh-A1) and adenosine deaminase (Ada-A2). mMdh-A1 was identified as non- diagnostic by Buth et al. (1987), however a different (obviously non-optimal) buffer was employed in the earlier study. The study of ADA products is often made difficult by the labile nature of the enzyme; it often loses activity rapidly upon frozen storage. PRELIMINARY DATA The original set of tissues sent to us included 13 specimens from Etter Pond (and a few additional specimens identified as hybrids). All thirteen specimens were either heterozygous for razorback and flannelmouth alleles at Ck-A or homozygous for the flannelmouth allele at this locus (Table 1). As found by Buth et al. (1987), this high level of introgression was not matched at the other diagnostic loci. Some additional introgressive heterozygosity was revealed at mMdh-A1, but none at mIcdh-A or Ada-A2. Whereas introgression at the Ck-A locus is evident in all populations of razorbacks, it is at a very low level compared 1