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Moran (1974), Long and Ballard (1976), Buynak and Mohr (1978), Jones et al. (1978), Loos et a1. (1978), Fuiman (1978, 1979), Wang and Kernehan (1979), McElman and Balon (1980), and Auer (1982). The pattern of three large lateral spots typical of early juvenile white suckers was recognized at least as early as Ellis (1914). Although Metcalf (1966) suggested that there is little rationale for subspecies designations (e.g., C. commer- soni suckeyi for western white sucker), diag- nostic comparisons of larvae herein are based on data from western populations rather than previous white sucker descriptions from east- ern populations. Winn and Miller (1954) included meso- larvae of razorback sucker, flannelmouth suc- ker, and Pantosteus species in their photo- graph-illustrated key. All Pantosteus larvae included in that publication have since been recognized as desert sucker ( Catostomus clarki) by Smith (1966). Pigmentation of bluehead and mountain sucker mesolarvae is typically similar to that documented by Winn and Miller for desert sucker. However, pig- mental variation is much greater and for bluehead and mountain suckers of similar size dorsal and lateral pigmentation can be indistinguishable from that illustrated and described by Winn and Miller for razorback sucker. In the process of documenting hybridiza- tion among several species of suckers, Hubbs et al. (1943) described young-of-the-year juveniles (and commented on some larvae) of flannelmouth, white, bluehead, and moun- tain suckers. Hubbs and Hubbs (1947) did the same for flannelmouth and bluehead suc- kers. Douglas (1952) published a photograph of a razorback sucker protolarva (or recently transformed mesolarva) without yolk. Doug- las's publication also included a photograph of a 10 cm specimen which, as noted by Winn and Miller (1954), is not a juvenile razorback sucker but rather an adult speckled dace (Rhinichthys osculus). Minckley and Gustaf- son (1982) chronicled the early development of the razorback sucker but their illustrations of larvae are poor and misleading. Mountain and Utah sucker larvae have not been des- cribed by authors other than ourselves. This publication covers six of seven species of Catostomidae inhabiting the Upper Colorado River System. The species not covered herein, the longnose sucker ( C. l catostomus), was described by Fuiman and Witman (1979) and included in the Great Lakes manual edited by Auer (1982). All seven species belong to the subfamily Catos- tominae and tribe Catostomini. Xyrauchen is a monotypic genus. Among the Catostomus species, the bluehead and mountain suckers represent a distinctive group known as "mountain suckers," subgenus Pantosteus; the others represent "valley suckers," subgenus Catostomus (Smith 1987). The biology and distribution of fishes in the Upper Colorado River System (Fig. 1) was most recently reviewed by Behnke et al. (1982), Carlson and Carlson (1982), Miller et al. (19813), Tyus et al. (1982), Woodling (1985), and Carlson and Muth (1989). Razorback, flannelmouth, bluehead, and mountain suckers are native to the System. The razorback sucker is protected by Colo- rado and Utah and has long been a candidate for the federal threatened and endangered species list. Flannelmouth and bluehead suckers are common to abundant in main- stem rivers throughout the System. The white sucker is present but less abundant in the Colorado, Gunnison, Yampa, and upper Green Rivers. The mountain sucker is res- tricted largely to headwater tributaries throughout much of the Green River Sub- basin and is rarely found in mainstem rivers. However, individual specimens have been reported in the Green River near the con- fluence with the Yampa River and in the White River near and above the confluence with Piceance Creek. The Utah sucker is restricted largely to headwater tributaries west of the Green River, particularly the Duchesne River. A few specimens have been reported in the Green River in or below the lower end of Dinosaur National Monument. The longnose sucker (c. catostomus) is found only above Flaming Gorge Reservoir, in the Blue Mesa area of the Gunnison River, and in the Lake Granby area of the Colorado River. The distribution and ecology of sucker larvae and young-of-the-year juveniles in the Upper Colorado River System has not yet been summarized, but selected information can be found in various progress reports, fmal reports, and formal publications by regional researchers (e.g., Carlson et al. 1979, Miller et al. 1982b, Haynes et al. 1985, and Tyus et al. 1987). 3