To assess the danger of Baylisascaris procyonis (the common roundworm of raccoons) to subhuman primates, as well as the zoonotic potential of the parasite, 4 squirrel monkeys were inoculated by stomach tube with 5,000 or 10,500 infective B procyonis eggs. Fulminating severe CNS disease developed at 10-15 days after inoculation, and the monkeys died or were euthanatized at 12-19 days. At necropsy, numerous hemorrhagic migration tracks were visible in coronal brain slices. Histologically, numerous migration tracks and larvae were seen in the brain as well as the spinal cord. Larvae measured 60-65 micrometers in midbody diameter. Migration tracks averaged 508 micrometers X 354 micrometers in cross section and consisted of foci of malacia and microcavitation, with an influx of macrophages and leukocytes, and various degrees of hemorrhage. Also seen were extensive perivascular cuffing, gliosis, astrocytosis, and a mixed inflammatory cell infiltrate. Numerous larval granulomas were seen grossly and microscopically throughout the body; they were particularly abundant in the tissues of the head, neck, and thorax. It was concluded that B procyonis should be considered a threat to the health of subhuman primates and an important potential zoonosis in situations wherein infection of monkeys or human beings could take place.