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PREFACE This publication is an expanded, updated, and retitled edition of our 1990 guide (Snyder and Muth 1990) to the larvae and early juveniles of six of seven catostomid fishes in the Upper Colorado River Basin (UCRB). Recognizing that morphological criteria for identification change dramatically as fish larvae grow and develop, and that diagnosis becomes especially difficult and complicated when species are very similar in appearance, the 1990 guide included 60 pages of keys, detailed descriptions (species accounts), and a comparative summary. For over a decade, that publication served well as a taxonomic reference for Larval Fish Laboratory, Colorado Division of Wildlife, Upper Colorado River Endangered Fish Recovery Program, San Juan River Basin Recovery Implementation Program, and other regional researchers. But species coverage was incomplete for the UCRB, new observations revealed the need to update certain descriptive data, and errors had been found in the printed keys, which also needed to be updated, expanded (for the seventh species), and, if possible, made easier to use. Longnose sucker (Catostomus catostomus) was not included in the 1990 guide because of budgetary limitations and the improbability of encountering its larvae or early juveniles in Recovery Program collections. However, with collection of juvenile longnose sucker and larvae suspected to be longnose sucker or hybrids in the lower Gunnison River in 1993, confidence in identification of those and other catostomids (including the progeny of reintroduced razorback sucker, Xyrauchen texanus) was compromised, and the need to comparably describe and incorporate the last of the UCRB catostomids in the keys became evident. To address this need and facilitate more accurate identification, larvae and early juveniles of longnose sucker were reared to supplement previously preserved developmental series, and their morphological development was documented in a new species account, a revised comparative summary, and a computer-interactive key which replaces the 1990 printed keys. As a modem alternative to long and intricate dichotomous or polychotomous keys, such as those in the 1990 guide, computer-interactive keys are much easier to prepare, update, and expand. They are also far more flexible for the user. Among other features, users can limit consideration to only likely candidate species, have available characters listed in the most diagnostic order for remaining candidates, and select from that list in any desired sequence-bypassing characters that are unfamiliar, difficult to assess, or based on structures that are damaged or missing. The new species account, comparative summary, and key, along with a list of corrections and other updates to the 1990 guide, were included in a manuscript for publication as a supplemental update (Snyder 2003). But rather than publish the supplement, along with a limited reprint of the 1990 guide, sponsors agreed that incorporation of the new and revised content in a new edition of the guide would be no more costly and considerably more desirable and convenient for users. IV