Stools from 1387 people were examined quantitatively for eggs of nematode parasites. The people were residents of Padangganting, near Sawahlunto, Sumatra (227 people), Sukamaju and Cibungur rubber plantations near Sukabumi, Java (831 people) and the villages of Mahima and Rabo near Reo, Manggarai, Flores (329 people). Ascaris lumbricoides, Trichuris trichiura and Necator americanus were common; Ancylostoma was not observed. Prevalence of parasitism was highest in Java and lowest in Flores. Egg counts were low, suggesting low worm burdens; intensity of infection with Ascaris and Trichuris was highest in Java, while hookworm was highest in Sumatra. Ascaris infections decreased and hookworm infections increased in prevalence and intensity with age; Trichuris was unaffected. A sex-related difference was observed only in Sumatra, where more females than males were infected with Ascaris. Village-to-village variation in prevalence of nematode infestation was observed in Flores and on the Sukamaju plantation; the topography of other areas did not allow for village comparisons. Ad hoc anthelmintic treatment of the residents of Cibungur plantation was reflected in reduced parasitism by Ascaris and Trichuris, but not hookworm, compared with neighbouring Sukamaju. Comparison of the patterns of disease in the three areas in terms of the occurrence of single, double or triple infections, revealed marked differences. In Java most people with parasitism had triple infections. In Sumatra the most common expression of parasitism was single infection with hookworm. In Flores single infection with Ascaris prevailed. It is suggested that the clinical effects of intestinal parasitism might depend on the pattern of infection. Samples collected in Flores were also examined for protozoa which were found in about half the population; Entamoeba coli and E. histolytica were most common.