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Differentiating between hatchery and naturally reproduced fishes is difficult because of the lack of appropriate marking techniques. Chemical immersion techniques can be a practical method for mass marking juvenile fishes. The objectives of this study were to determine the concentration of oxytetracycline (OTC) hydrochloride needed to effectively mark age-0 yellow perch Perca flavescens to observe the retention lime of the mark, and to measure the persistence of OTC in body tissues. Fish were immersed in 309, 534, and 748 mg OTC/L for 12 h. Initial fish collections were made weekly, and monthly collections began 1 month postmarking for tissue and mark analyses. High-pressure liquid chromatography was used to quantitate OTC residues in tissues; OTC concentrations were undetectable (<0.05 µg/g) in the edible tissue (muscle) at 110 d postimmersion. Aided by a Nikon Labophot fluorescence microscope, we discerned otolith marks 56 d postimmersion on 100% of the fish treated at 534 and 748 mg/L. Marks were still visible on 100% of the fish treated with 534 and 748 mg/L at 8 months postimmersion.