Seven different species of filarial parasites were found in the eight species of monkeys which occur in Panama, 87 per cent of the 72 wild monkeys which were examined being infected with one or more species of filaria. Acanthocheilonema gracile was the parasite most frequently encountered, the adult worms being located in the serous cavities of the red and black spider monkeys and the white-faced monkey. The microfilaria of this species possessed a sheath; all other microfilariae were unsheathed. Two new species belonging in the genus, Tetrapetalonema Faust 1935, have been described. One of these, T. atelensis, was found beneath the fascia of the back muscles of the red and black spider monkeys; the other, T. parvum, was found in connective tissue between the back muscles of the white-faced and yellow titi monkeys. Two new species of microfilariae, M. panamensis and M. obtusa, have been described from these two latter hosts. The adult forms of these larvae were not found.
The writer is indebted to Dr. Herbert C. Clark, Director of the Gorgas Memorial Laboratory, for providing the facilities for making the investigation, and to Dr. E. G. Hakansson and Dr. Carl M. Johnson for aid in the collection of material.