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The large, fan-shaped, first interneural bone of razorback sucker metalarvae and juveniles over 16 mrn SL readily distinguishes it from the other species (Fig. 8). By late in the metalarval phase, the smaller interneurals posterior to the first also develop enlarged or flared tops. The intemeurals eventually form the skeletal basis for the unique predorsal keel or "razor" of the razorback sucker (Fig. 94). By 20 mrn SL, the first interneural generally segregates the remaining species according to subgenera. Most members of subgenus Catostomus (at least Utah, white, and flannelmouth suckers) have moderate to large anvil-shaped first interneurals with moderate to long posterior extensions (especially long in flannelmouth sucker). Subgenus Pantosteus (bluehead and mountain suckers) have smaller, somewhat blocky first interneurals with short to moderate posterior extensions. The interneurals for similar-size longnose sucker (also subgenus Catostomus) examined for this study are less well defined and appear to develop more slowly than for the other species. The small size and abbreviated shape of the first interneural in longnose sucker juveniles about 40 mrn SL (Fig. 107) appears more like those of subgenus Pantosteus metalarvae or juveniles about 21-22 mrn SL (Figs. 8, 52, and 80) and is perhaps associated with the more cylindrical anterior body shape of longnose sucker than the other members of subgenus Catostomus. The position of mandibles relative to maxillae is also diagnostic for subgenus Pantosteus. For juveniles and metalarvae greater than 22 mrn SL, the anterior margins of the mandibles are closer to the posterior than anterior ends of the maxillae in bluehead sucker and mountain sucker (Fig. 9). For the other species, they are closer to the anterior ends of the maxillae. However, by about 40 mrn SL, at least some flannelmouth suckers have anterior margins of the mandibles positioned about midway between anterior and posterior ends of the maxillae. Shape and size of anterior-dorsal projections on the maxillae are diagnostic for razorback sucker and subgenus Pantosteus greater than 22 mrn SL, sometimes smaller. The anterior-dorsal projections of the maxillae are very shallow to almost absent in razorback sucker, relatively long and pointed (at least as deep as wide at the base) in bluehead and mountain suckers, and intermediate (prominent but blunt and less deep than wide at the base) in subgenus Catostomus (Fig. 10). By 40 mrn SL, these projections grow but relative differences in size and form continue with those of Pantosteus and most Catostomus projecting forward (Fig. 51) or even a bit outward (Fig. 37). In contrast, the anterior-dorsal projections of the maxillae of longnose sucker grow a bit larger than other members of subgenus Catostomus and project forward and uniquely inward or medially (Fig. 107), perhaps facilitating development of a somewhat longer, more conical snout. The angle at which the postcleithrum extends from the cleithrum was initially suspected to be diagnostic for subgenus Pantosteus, about 900 for bluehead and mountain suckers and variable, but usually much less angled for the others (Fig. 11). However, the differences in this character are not always distinct, and perceived postcleithral angle can be affected strongly by angle of view. Computer-Interactive Key Introduction. - Covering bluehead, flannelmouth, longnose, mountain, razorback, Utah, and white suckers, the "Computer-Interactive Key to Eggs, Larvae, and Early Juveniles of Catostomid Fishes in the Upper Colorado River Basin," provided as part of this report either on CD (compact disk in pocket on inside rear cover of printed copies) or by download over the Internet (see instructions below), is an updated and expanded replacement for the printed keys in the 1990 guide (Snyder and Muth 1990). It is a data set of 110 characters and 234 taxon items 35