The sensitivity in vitro to metronidazole and ornidazole of 7 stocks and of the cloned lines of 5 stocks of Giardia isolated from humans, rodents and monkeys was studied by the growth inhibition test. All 7 stocks of Giardia, irrespective of the host, differed in their sensitivity to these drugs, commonly used in therapy of human giardiasis. The differences were greater with ornidazole than with metronidazole. The 5 Giardia stocks from which clones were prepared were found to consist of populations with significantly (P less than 0.05) differing sensitivities to ornidazole and metronidazole. There was a positive correlation between high resistance in vitro to both drugs of all clones of one parent stock and treatment failures of giardiasis in the patient from which the parasite stock had been isolated. The spectra of sensitivity of Giardia to anti-giardial drugs may have implications concerning the suspected zoonotic character of human giardiasis.