The purpose of the present study was to evaluate the effect of chloroquine treatment on cyst excretion in calves naturally infected with Giardia duodenalis. The calves were randomly assigned into two groups based on placebo (group I, n=7 untreated control calves) or treatment (group II, n=7 calves treated orally with 2.5 mg/kg chloroquine twice daily for five consecutive days). The G. duodenalis isolates were identified by molecular characterization with beta-giardin nested PCR and gene sequence analysis as assemblage A3. Cyst excretion was determined on days 0, 3, 7 and 10, before and after treatment. Geometric means of the number of excreted cysts did not change significantly in the control group during the trial. The reduction in cyst excretion after chloroquine treatment was 99% on day 3 and 100% on days 7 and 10. Chloroquine treatment is most probatly practically applicable, relatively inexpensive and highly effective against giardiosis in calves.