Protozoans of the family Opalinidae are intestinal commensals in amphibians. To test the hypothesis that these organisms are susceptible to the antiprotozoal antibiotic metronidazole, we randomly assigned 60 juvenile Woodhouse's toads (Bufo woodhousii) to receive a single oral dose of metronidazole or water. In pilot trials, the prevalence of opalinids in untreated members of this population was over 70%. One-third of the study population was dissected at each of 3 time points: 18 hr, 1 wk, and 2 wk post-treatment. An examiner blinded to the toad's treatment history determined the presence or absence of opalinids using a dissecting microscope. Opalinids were found in 3/10 toads in the treatment group and 9/10 in the control group after 18 hr (P < 0.02), in none of the treatment group and 8/10 in the control group after 1 wk (P < 0.001), and in none of the treatment group and 10/10 in the control group after 2 wk (P < 0.0001). These results suggest that a single-dose of metronidazole quickly and reliably clears opalinids from juvenile Woodhouse's toads with no evidence of short-term recurrence. The treatment was well tolerated, with no apparent morbidity and no mortality in either group. Future exploration of opalinid-related host fitness consequences may be facilitated by this simple method of developing a protozoan-free host population.