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I t INTRODUCTION The Colorado squawfish (Ptychocheilus lucius) is the largest cyprinid native to North America. It is endemic to the Colorado River basin and his- torically was widely distributed in the Colorado River and its major tribu- taries. Following major water developments on the Colorado River system, squawfish populations declined and the species is now listed by the U.S. Fish and Wildlife Service as endangered (USFWS, 1974). The Colorado squawfish, a predator, historically had only soft-rayed fish as prey. Early in this century the channel catfish (Ictalurus punctatus) was established in the Colorado River system as a sport fish. Since that time, there have been undocumented reports of Colorado squawfish found dead with channel catfish lodged in their esophagus (Seethaler, 1978; Vanicek and Kramer, 1969). The only documented evidence of this phenomenon was reported by McAda (1983) who collected an adult Colorado squawfish (550 mm TL) with a channel catfish (120 mm TO lodged head first in its esophagus, posterior to the gill arches. There has been some concern that the lodging of juvenile channel cat- fish in throats of Colorado squawfish could be one of the causes of the de- cline in numbers of this endangered species. Hence, a laboratory experiment was set up to study the phenomenon in more detail. Objectives of the study were to 1) determine if adult Colorado squawfish would prey on channel cat- fish without prior conditioning, 2) determine if channel catfish could be swallowed or tend to lodge in the esophagus, and 3) determine if Colorado squawfish are injured in the process of trying to eat channel catfish.