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NorthWest Marine Technology, Incorporated!, Bonneau et al, (1995) used this technique to mark salmonids, Dewey and Zigler (1996) to mark bluegill Lepomis macrochirus, and we used it to mark small (30-70 mm TL) Colorado squawfish (Haines and Modde 1996), A successful batch mark must have the capability of providing several different marks and last for one or two growing seasons, This study compared the effectiveness offreeze branding and elastomer injection for use in short-term identification of family lots of juvenile razorback sucker and Colorado squawfish, Specifically, we addressed three objectives: 1) to compare freeze branding and visible implant fluorescent elastomer tags for rate of retention during a 15 month period, 2) to determine if the four available colors of elastomer are equally detectable, and 3) to compare the operculum (razorback sucker) or lower jaw (Colorado squawfish) and base of the anal fin as locations for an elastomer implant. Methods The freeze branding apparatus used in this study was similar to one described by LaJeone and Bergerhouse (1991). It contained a 2 Lreservoirof liquid nitrogen and a metal branding iron extending through the wall of the reservoir, The branding iron terminated in a V -shape, The left side of the fish, anterior to the origin of the dorsal fin, was held against the iron for approximately 6 s for marking, Four colors (green, blue, red, and orange) of fluorescent elastomer, obtained from Northwest Marine Technology, Incorporated, were injected subcutaneously with a 1.0 ml insulin syringe and 28-gauge needle to one of two locations on the fish, 1 Mention of trade names or commercial products does not constitute endorsement by the Recovery Implementation Program for Endangered Fish Species in the Upper Colorado River Basin. 7