The examination of thick blood films from 5,000 persons from various sections of the city of Belem (State of Par?), Brazil, revealed that 541 (10.8 per cent) of thids sample harbored microfilariae. Elephantiasis was observed in 1.3 per cent of the persons examined. The percentage of Filariasis (positive blood specimes plus cases of Elephantiasis) was 12 per cent. The morphology and periodicity of the microfilariae identified the species as Wuchereria bancrofti (Cobbold 1877), Seurat, 1921. No other species of human Filaria was found during this survey. An analysis by age groups showed an increasing incidence of microfilariae with age. The youngest individual with microfilariae in the blood was two years old boy, and the youngest person with Elephantiasis was an 11 years old boy. The principal vector was found to be Culex fatigans. Among 1,014 dissections, 118 specimens (11.6 per cent) were positive for Filaria larvae. Anopheles darlingi and A.aquasalis were also found naturally infected in Belem. Experimental infections were produced in the following species: Culex fatigans, Anopheles darlingi, A.aquasalis, A.oswaldoi, A.triannulatus and A.albitarsis
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