Summary

• 1.  Man, cattle, and horses in the onchocerciasis zone of Guatemala are infected with microfilariae which inhabit the dermal layer of the skin and do not circulate in the blood. These microfilariae belong to the species Onchocerca volvulus (Leuckart, 1893), O. gutturosa Neumann, 1910, and O. reticulata Diesing, 1841 (= O. cervicalis Railliet and Henry, 1910), respectively.
• 2.  The microfilariae of these three species can be distinguished morphologically on the basis of the form, size and arrangement of the caudal nuclei.