1. (1) Yellow fever neurotropic vaccine administered by scarification was used in the control of an epidemic of yellow fever in Southern Nigeria, 1951–1952. 2. (2) Reactions to the vaccine were common and are classified as mild, viscerotropic and neurotropic. 3. (3) The mild reactions were characterized by fever and headache. 4. (4) Viscerotropic reactions were very rate and were differentiated from the mild reactions by the presence of albuminuria and jaundice. 5. (5) Viraemia was demonstrated in patients from these two groups, but the circulating virus was of low titre. 6. (6) Neurotropic reactions were characterized by encephalitis with a high mortality rate. 7. (7) The most conspicuous lesions in the brain were small areas of focal necrosis, cuffing of the blood vessels and congestion. 8. (8) Lesions were also noted in the liver and spleen, but these are considered to be non-specific. 9. (9) Yellow fever vaccine virus was isolated from brain sections.