The efficacy of hydrogen peroxide in controlling mortality associated with external columnaris infections on walleye Sander vitreus (formerly Stizostedion vitreum) and channel catfish Ictalurus punctatus fingerlings was evaluated in two on-site trials. Microscopic examination of skin scrapes before treatment confirmed the presence of bacteria indicative of columnaris disease. In separate trials, walleyes (12 g) and channel catfish (2.3 g) were exposed to a total of three every-other-day hydrogen peroxide treatments of 0, 50, 75, or 100 mg/L for 60 min. Each treatment regimen was tested in triplicate, and each replicate contained 13 walleyes or 26 channel catfish. Hydrogen peroxide exposures of 50 mg/L for 60 min significantly increased walleye survival relative to that at other treatment concentrations, including the controls. Exposures of 50, 75, or 100 mg/L of hydrogen peroxide for 60 min significantly increased channel catfish survival in comparison with that of controls. These trials indicated that hydrogen peroxide treatments effectively reduce mortality in walleyes and channel catfish infected with external columnaris.