Summary 1. A red howling monkey, Alouatta seniculus, was infected with yellow fever virus by mosquito bites. Its temperature reached 104°F. on the third and fourth days. Virus was passed back successfully to M. rhesus by direct transfer of blood and by the bites of mosquitoes fed upon the howler during its febrile access. Convalescent serum from the howler protected perfectly against virus in a test animal. 2. Two monkeys, tentatively identified as Callicebus moloch, were inoculated with yellow fever virus. One developed protective bodies in the blood without ever having shown fever. The other had fever on the fifth day after injection; rhesus monkeys were infected from this animal both by blood transfer and by mosquito transmission. 3. An owl monkey, Aotus trivirgatus, was fed upon by infected Stegomyia mosquitoes. The animal had no subsequent rise in temperature, but developed immune bodies in the blood, as determined by protection against virus in a test rhesus. 4. The red-faced monkey, Cacajao rubicundus, proved resistant to yellow fever virus. The serum of C. rubicundus 3 protected against virus both before and after the inoculation of infectious material. 5. An attempt was made to infect two saki monkeys, Pithecia monacha, the first with blood virus, the second with mosquito virus. The temperature of the second animal reached 104°F. on the fourth day. Blood transfers back to M. rhesus produced infections in both cases. Mosquitoes became infective after feeding on P. monacha 2. The convalescent serum from neither proved highly protective against virus in rhesus monkeys; the test animal for P. monacha 1 recovered after a severe febrile course, that for P. monacha 2 died with yellow fever on the twenty-fourth day, its temperature never having reached 104°. 6. One Cebus variegatus was infected by mosquito bites. It showed an irregular febrile reaction, similar to that noted in C. frontatus. Virus was transferred back to M. rhesus by blood inoculation and by mosquito transmission, producing fatal infections in both cases. The convalescent serum of this C. variegatus was strongly protective against yellow fever virus. 7. A spider monkey, Ateleus ater 5, was inoculated with yellow fever virus. A positive transfer was made to M. rhesus on the fourth day. However, no protection against virus was obtained from 3 cc. of convalescent serum. 8. Pithecia monacha 1 and Ateleus ater 5 were reinoculated with virus as an immunity test. From neither was it possible to obtain virus from the blood stream on the fourth day thereafter.