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to use tagging and marking systems, methods of tagging, and associated handling procedures which reduce adverse effects on these fish while providing the greatest amount of information. The Carlin-dangler tag was used extensively on wi]d and hatchery-reared endangered fishes in the Colorado River basin from 1979 to 1986~(Valdez et al. 1981; Miller et al, 1982; Archer et al. 1985). These species included the endangered Colorado squawfish (Ptvchocheilus Lucius), humpback chub (Gila cvpha), bonytail (G. ele ans), and razorback sucker (Xvrauchen texanus). However, critics of this tag expressed concern that it was unsatisfactory for marking endangered fishes because of its poor tag retention on long-lived fish, and more importantly, it was injurious to fish (e. g., punctures made by needles that were used to attach the tag caused open wounds and secondary infections). Because of these concerns the use of this tag to mark endangered fish in the UCRB was suspended in 1989. A suitable identification system was needed to replace the Carlin-dangler tag, one that would improve fish tagging methods by reducing or eliminating problems associated with previous tagging and identification systems. In 1987, studies (U. S, Fish and Wildlife Service [USFWS] 1990) were conducted to evaluate the passive integrated transponder (PIT) tag. This study evaluated the performance of PIT tags implanted in adult roundtail chub, immature Colorado squawfish, and immature razorback sucker. Although only limited data were obtained because few fish were recovered from riverside ponds, the short-term performance of this tag and its interrogation system demonstrated merit for field and laboratory use. This study (USFWS 1990) recommended continued evaluation of the performance of PIT tags and conducting new evaluation studies for other tagging and marking systems on various sizes of Colorado squawfish, razorback 2