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7. Confidence in identification is high except among the larval and early juvenile catostomids of the Yampa River, which are complicated by extensive hybridization. The identity of the plains killifish (Fundulus kansae) collected in the Colorado River trend zones is being verified by Dr. R. R. Miller of the University of Michigan; Holden and Stalnaker (1975) identified similar fish collected between these trend zones and in the Gunnison River as the Rio Grande killifish (F. zebrinus). The Yampa River specimens are defin- itely plains killifish. Characteristics used to distinguish larval and early juvenile Colorado squawfish from the chubs (Gila sp.) are based on a limited series of squawfish provided by Karl Seethaler of Utah State University and numerous collected roundtail chubs (Gila robusta). However, the squawfish series is derived from only one or two broods reared at the Willow Beach Hatchery in Arizona and accordingly, genetic variability is restricted and the characters used may not apply to all Colorado squawfish. Separation of the larvae and early juveniles of the endangered humpback chub, and the very rare bonytail chub (Gila elegans) and the roundtail chub (G. robusta), as well as the rare humpback or razorback sucker (Xyrauchen to anus) and the flannelmouth sucker (Catostorus latipinnis), is based on extensive reference series of the roundtail chub and flannelmouth sucker and certain adult charac- teristics, particularly fin ray meristics. Known larval or early juvenile reference specimens of the rare or endangered chubs and sucker are not yet available for comparative study. Fish collections were typically made at or near access points at the beginning and end of each trend zone. The remaining trend zone sampling sites were selected on the basis of potentially favorable and diverse habitat while slowly canoeing downstream. Floating the trend zones with rafts as originally planned would have been difficult if not impossible due to extremely low flows. At each collection site generalized habitat types were