Babesia species are tickborne hemoprotozoans. Although experiments have shown that primaquine is highly effective in the treatment of Babesia species infections in mammals, this drug has not been widely used for the treatment of avian babesiosis. Consequently, the treatment of this disease for avian patients has traditionally relied on an empirically established imidocarb treatment. In this study, the authors examined the efficacy of primaquine as a treatment alternative for avian babesiosis (Babesia peircei and Babesia ugwidiensis) in seabirds. Retrospective analysis was performed on the medical records and blood smears of 446 B peircei–positive African penguins (Spheniscus demersus) and 41 B ugwidiensis–positive Cape cormorants (Phalacrocorax capensis) admitted for rehabilitation at the Southern African Foundation for the Conservation of Coastal Birds (SANCCOB, Cape Town, South Africa). Treatment with primaquine (1 mg/kg PO q24h for 10 days) was effective in rapidly and markedly decreasing the proportion of Babesia–positive blood smears in African penguins and Cape cormorants. No regurgitation, loss of appetite, or any other signs after administration of primaquine were observed during the study period. The use of primaquine can be a particularly advantageous treatment alternative for avian babesiosis in circumstances in which it is not possible to determine confidently whether the intraerythrocytic inclusions seen in blood smears correspond to Babesia or Plasmodium or in cases in which there is a coinfection by Babesia and Plasmodium.