Abstract 1. 1) At the outbreak of World War II, Brugia malayi was known to cause endemic filariasis in rural areas of Malaya, and a microfilaria resembling B. malayi had been found in macaque monkeys. W. bancrofti infections had been seen only in immigrants, and were thought to have been imported. Post-war investigations have changed this picture considerably. 2. 2) A number of small foci of periodic W. bancrofti infection have been discovered, one urban and several rural. In one rural area the vector was an anopheline, and the parasite developed poorly in local Culex fatigans, which however was a good host for an urban strain of W. bancrofti from Singapore. 3. 3) Two forms of B. malayi have been recognized in man. The periodic form is transmitted chiefly by the open-swamp species of Mansonia and by some anophelines; although not confirmed as a natural infection in animals, it can be transmitted to them experimentally. This form of B. malayi appears to be the common one in other parts of Asia. The chief vectors of the semi-periodic form are the swamp-forest Mansonia, and natural infections in animals are common; transmission from man to animal is relatively easy, and in parts of Malaya this form almost certainly behaves as a zoonosis. In the same areas animal infections with another Brugia, B. pahangi, are common; although not recognized as a natural infection in man, it has been transmitted to him experimentally. This situation has not yet been observed in other countries of Asia. 4. 4) Charts illustrate infection rates by age and sex, and the usual site of elephantiasis, the relationship between infection rate and microfilaria density, and the results of control measures. Despite very high microfilaria rates in young children of both sexes, females in the semi-periodic B. malayi area showed a much lower infection rate than males from the age of 10 years onwards. Repeated surveys in untreated populations showed the importance of natural fluctuations in incidence, but also showed that many people could live for years in highly endemic areas and remain apparently free from infection. 5. 5) Control was based largely on once-weekly doses of 5 mg. diethylcarbamazine citrate/kg./body wt. given for 6 weeks. Results were better and more lasting where the periodic B. malayi was prevalent and no animal infections were known. 6. 6) The broader significance of these findings is discussed and possible lines of investigation are suggested.