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Colorado squawf ish from the middle Colorado River reach (RM 0-100) were first implanted with radio transmitters in fall 1982 when several fish were tagged with transmitters that had a theoretical 18-month battery life. However, because the duration of radio contact with these fish and with those implanted with similar transmitters in the upper reach (RM 125-200) only lasted about 9 months, we suspect that the actual Life of the transmitters was much less than the 18 months reported by the manufacturer. Transmitter failure is the most likely explanation for _ our inability to make radio contact later than summer 1983 with the Colorado squawf ish implanted with 18-month transmitters in fall 1982. A mature-size Colorado squawf ish (No. 23; Table 4) from the Gunnison River was implanted with a radio transmitter and released at RM 33.7 on 10 September 1982. Subsequent radio contacts with this fish, all in the Gunnison, were made at the following locations; RM 31.5, 21 October 1982; RM 30.5, 27 October 1982; RM 33.8, 16 August 1983; RM 30.4, 25 October 1983. For this particular fish no extensive movement within the Gunnison River was observed. Humpback and roundtail chub-Radio tracking humpback chub in the Colorado River provided only limited information on movement and spawning. Radio contact was maintained for a significant period with four of the ten adult humpback chub implanted with radio transmitters in the Black Rocks area. Contact was lost with two fish when they were released, whereas radio contact with four fish lasted no more than 2 weeks (Figure 9). Two of four chub with which radio contact was maintained exhibited no displacement from their release location. Of the remaining two fish, one fish exhibited limited movement (± 0.3 mi) near the release location whereas the second moved more than one mile downstream -- outside of the Black Rocks area -- sometime later than 1 month after its release. Three of the four radiotagged humpback chub with which we had radio contact during the Late-June spawning period were found near the locations where they were released -- deep-water areas among igneous rock formations. These observations suggested that humpback chub spawning occurred near these habitats. Mature roundtail chub were co]_lected from these same areas at that time. Waters 10-1.5 m deep occur in several parts of the Black Rocks area. However, controlled experiments performed recently in Black Rocks revealed that radio contact cannot be made with transmitters deeper than about 5 m. Therefore, radiotagged fish that regularly utilize deep-water habitats might not be detected by our equipment. This might explain our inability to make radio contact with some fish after they were released. r 19