The authors studied the prophylactic activity of long-acting oxytetracycline in the gerbil, Meriones unguiculatus, infected with the cattle piroplasm Babesia divergens. Parasitemia, packed cell volume (PCV) and survival rate were monitored. At 5 and 20 mg/kg, the prophylactic activity was absent and all the animals died. At 40 mg/kg, two animals out of five survived but were clinically affected and sustained a severe hemolytic anaemia. However, at 80 mg/kg, the parasitemias remained very low, the PCV's were unchanged and all the animals survived; furthermore, they were totally resistant to a challenge infection given 33 days after the primary infection. There is a good agreement between these results and those reported in cattle naturally or experimentally infected with B. divergens.