Hydrogen peroxide (H2O2) and metrifonate (Mtf) are common products used in ectoparasite infestations on fish cultures. The therapeutic efficacy of H2O2 and Mtf on a common monogenean parasite, Ligictaluridus floridanus, was evaluated in channel catfish (Ictalurus punctatus). In vitro trials were conducted using excised fish gills naturally infected with L. floridanus, which were immersed in H2O2 (150, 300, and 570 mg L−1) and Mtf (0.25, 0.5, and 0.75 mg L−1) solutions. The efficacy of the treatments was based on the survival time of the parasites, observed microscopically. In addition, an in vivo trial using catfish juveniles, naturally infected with L. floridanus, was also performed. One group received immersion baths of 570 mg L−1 H2O2 (3%) during 4 min; the Mtf (90%) group received 0.5 mg L−1 Mtf for 10 min. Treatments were done on days 3, 7, and 11 of the experiment. Results indicate that baths with Mtf do not significantly reduce the mean intensity of the parasite per gill arch, nor do they reduce the in vitro survival time of parasites during treatment; H2O2 baths at 570 mg L−1 during 4 min were effective (P < 0.05) against adult and juvenile stages of L. floridanus. This study supports the use of H2O2 as an effective antiparasitic agent against I. punctatus.