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. . Radiotelemetry. Radiotelemetry was used to locate and follow movements of adult Colorado squawfish and razorback suckers. Twenty radiotransmitters (fish modules), were provided by the FWS or use in this investigation; 10 manufactured by Smith-Root (P-40 type) and 10 manufactured by AVM (SM-1 type). The Smith-Root and AVM transmitters were powered by lithium and mercury batteries, respectively, with a 12 to 14-month life expectancy. These transmitters operated within the 40.660 to 40.700 MHz range, with pulsed transmission rates of 40 to 80 counts per minute. All transmitters were tested for pulse and frequency by FWS upon receipt from the factory, and each was dipped in melted beeswax prior to implantation. Pulse and frequency were checked again by BIO/WEST prior to and immediately following implantation to insure that the transmi tters were performing according to factory specifications. . . Radio receivers used were Smith-Root search (SR-40) and standard tracking (RF-40) types provided by FWS. An Advanced Telemetry Systems (ATS) programable scanning receiver provided by BOR was also used periodically. Antennas used were Larsen Kulrod NMQ-40 whip antennas and Smith-Root loop antennas. . Implanting Radio Transmitters . All adult Colorado squawfish and razorback suckers captured while electrofishing were transported from the capture location to camp (generally less than 2 miles) for weighing and measuring. untagged fish were tagged with a uniquely-numbered green or orange Carlin tag and if large enough, equipped with a radiotransmitter. Only squawfish 530 rom total length (TL) or larger were implanted with the exception of one individual, 510 rom TL, that was judged to be sufficiently robust to accept a transmitter. Fish too small to be implanted were tagged and released. All adult razorback suckers were implanted. . The radiotransmi tters were implanted surgically according to the techniques described by Tyus (1982). Fish were anesthetized by introducing tricaine methanosulfate (MS-222) to the live well. A fish were determined to be sufficiently anesthetized when, within 1-2 minutes, it would lose equilibrium in the live well. Surgery was done by placing the fish on a fish measuring board outside of the live well. One person held the fish firmly in position while another continuously bathed the gills with anesthetic water from the live well to a point about halfway through the surgery at which time fresh water was used to bath the gills. A small incision (2.5-3 em long) was made anterior and slightly dorsal to the insertion of the left pelvic fin. The transmitter was inserted toward the posterior end of the fish so it came to rest on the pelvic girdle within the parietal cavity. The incision was closed wi th four to eight sutures. The fish was then observed in the live well with fresh water for a period of 5 to 15 minutes until it was actively swimncing and was then released. . . . 7 .