In a study of 210 people from all age groups in the Venilale District of East Timor, 49% had Ascaris lumbricoides, 1% Trichuris trichiura and 67% hookworm infection. There were high Ascaris infection rates among some of the children, but the Trichuris and hookworm rates were almost uniformly low. The factors responsible for these rates are enumerated, and the complex interaction of the factors is discussed. The relative lack of shade and the well-drained limestone soil are probably in large measure responsible for the generally low helminth prevalence. Ascaris was the principal cause of eosinophilia. It was not possible to relate any other haematological or serological findings to the helminths. Apart from the high Ascaris infections in young children, it is not thought that these helminths constitute a heatlh problem.