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Executive Summary Collections offish larvae and early, young-of-the-year juveniles are necessary to help determine fish spawning sites and seasons, and assess larval production, transport, distribution, nursery habitat, and survival, as well as other aspects of early life history. But use of such collections requires accurate identification of the target species. The morphological appearance of all similar-appearing species in the waters sampled and diagnostic criteria for distinguishing the target species from them must be known. Morphological criteria for identification change dramatically as fish larvae grow and develop, and when species are very similar in appearance, as in the case of many cypriniform fishes (minnows and suckers) in the Upper Colorado River Basin (UCRB), diagnosis becomes especially difficult and complicated. Still, the 1990 Colorado Division of Wildlife (CDOW) guide to the larvae and early juveniles of six of seven suckers (Catostomidae) in the UCRB (Snyder and Muth 1990) has served CDOW, the Upper Colorado River Endangered Fish Recovery Program (Recovery Program), and other regional researchers well. But species coverage is incomplete, some descriptive data need to be updated, and the very long and intricate set of printed keys needs to be corrected for the occasional error, updated and expanded (for the seventh species), and made more flexible and user friendly. The goal of this project was to facilitate more accurate identification of the larvae and early juveniles of western Colorado and UCRB suckers with a primary objective of updating and completing the 1990 guide (Snyder and Muth 1990). Accordingly, this final report is a supplement to the 1990 guide and serves as a manuscript for more formal subsequent publication as such or a revised and updated edition of the guide. It includes updated descriptive information for previous species accounts, a comparable new species account for longnose sucker, an updated and expanded comparative summary, and a new computer-interactive key (program and files on compact disk or downloadable via the Internet) to correct, update, expand, and replace the existing printed keys. A secondary objective of the project was to provide proof- of-concept for the effective application of computer-interactive keys to fish larvae. Both objectives were successfully accomplished. An interim descriptive species account with detailed drawings was prepared for longnose sucker and an effort to rear a new developmental-study series of the species was begun as an associated investigation sponsored by CDOW (Snyder 2001). With some adjustments for use of computer imaging technology, methods for developmental study oflongnose sucker, including analysis of morphometric, meristic, pigment, and developmental-state characters; clearing and alizarin-red staining of whole specimens for skeletal study; illustration; and data analysis and summarization generally followed Snyder and Muth (1990). The species account was based on a previously reared, but abbreviated series of larvae from an east-slope population and positively identified juveniles collected from the Gunnison River. The new culture effort, also based on east-slope stock. and description of the species were completed with this project. In addition to the completed species account, descriptive data for longnose sucker also were used in the updated and expanded "Comparative Summary" and "Computer-Interactive Key." Through the years since publication of the 1990 guide (Snyder and Muth 1990), mistakes in that guide were noted by the authors, reviewers, and users; character-range extensions for most described species were recorded by Larval Fish Laboratory (LFL) and other researchers; and for white sucker C. commersoni, better drawings for two larval stages became available. These, with appropriate amendments, were compiled in a list of errata and updated descriptive 8