The aim of the present study was to evaluate the efficacy of chloroquine on lessening or eliminating cyst shedding in dogs naturally infected with Giardia duodenalis. A total of 26 dogs naturally infected with G. duodenalis, from various breeds, ages and of both sexes were selected and randomly assigned into two groups based on treatment (group I, n = 20 dogs treated orally with 2.5 mg/kg chloroquine twice daily for 5 consecutive days) and control (group II, n = 6 untreated control dogs). Diagnosis was based on microscopic faecal examination and rapid test kits. Cyst excretion was determined on days 0, 3, 7 and 10, before and after treatment. Evaluation of the percentage reduction in cyst excretion revealed 99.8% efficacy on day 3 and 99.9 % on days 7 and 10 in the treatment group. Geometric means of the number of excreted cysts did not change significantly in the control group. Chloroquine at the proposed dosage might be a practical in application, low priced and highly effective treatment option in dogs with giardiasis.