A battalion of Royal Thai Marine militia was assigned to take either 50 mg or 100 mg of doxycycline daily or pyrimethamine/dapsone weekly for malaria prophylaxis on the Thai-Kampuchean border for a 17 week period. Attack rates for the groups expressed as cases/100 men were 34 for 50 mg doxycycline, 18 for 100 mg doxycycline, and 52 for pyrimethamine/dapsone. The relative efficacy of the two doxycycline regimens compared to Maloprim were 1.6 and 1.4. Compliance with the daily drug nearly equalled that of the weekly regimen. This suggests that 100 mg of doxycycline daily can be effectively used for malaria prophylaxis by soldiers under operational conditions on the Thai-Kampuchean border.