The course of Plasmodium inui and Babesia microti infections was studied in seven splenectomized squirrel monkeys (Saimiri sciureus) of Guyanan or Bolivian origin. Three of the monkeys were infected with P. inui either by the inoculations of parasitized blood or by the bite of infected mosquitoes. The remaining four monkeys were infected by the inoculation of parasitized blood, containing P. inui and B. microti in three and with B. microti only in one. The infection in all seven animals was severe, terminating fatally.