The safety of 5% ponazuril oral suspension was evaluated in two-week-old Beagle puppies by administration of 250 mg ponazuril/kg for three consecutive days. Exaggerated dosages had no adverse effect on health or pathologic (clinical, gross, micro) indices. In two studies to evaluate the efficacy of ponazuril against naturally acquired C. ohioensis-like infections, Beagle puppies were ranked by oocyst counts, randomly allocated to treatment or control groups, and general health observations and oocyst counts were recorded daily for 14 days post treatment. In study 1, 25 Beagle puppies were allocated to five groups: ponazuril 20 or 30 mg/kg once (day 0) or twice (days 0 and 7) or placebo given twice. One treatment at 30 mg/kg significantly reduced oocyst counts (P < 0.019), and retreatment with either dosage significantly increased efficacy (P < 0.05). In study 2, 30 puppies were allocated randomly to six groups: ponazuril 20, 30, 40 or 50 mg/kg once (day 0), or ponazuril 30 mg/kg or placebo twice (days 0 and 7). Oocyst counts were significantly reduced (P < 0.02) by 30 or 50 mg/kg, and dosages ≥ 40 mg/kg acted more rapidly. No adverse effects of ponazuril were observed in either study, and no significant differences in clinical signs of coccidiosis were observed.