In a group of rural school children in the highlands of Papua New Guinea with an initial prevalence of Ascaris lumbricoides 71 per cent, Trichuris trichiura 69 per cent and Necator americanus (hookworm) 95 per cent, 2 1/2 months after treatment with mebendazole the prevalence of A. lumbricoides and T. trichiura was very low, that of N. americanus was about 20 per cent and all egg counts were very low. Prevalence rates and egg counts of A. lumbricoides returned to pretreatment levels 9 months after treatment. Prevalence of T. trichiura and N. americanus, one year after treatment, was lower than before treatment and 97 per cent of egg counts were low. Two years after treatment, prevalence of all species was normal, but mean egg counts of N. americanus were about half of pretreatment levels. Thus in areas with comparable transmission rates, annual treatment with a course of anthelmintic would keep hook-worm levels low, and an additional dose at mid year would also considerably reduce ascariasis. Presence or absence of A. lumbricoides in particular subjects varied from one examination to another, and a particular child was not more likely to have the same infection status one year later, given the prevailing infection rate. 95 per cent were positive for A. lumbricoides on one or more of three examinations held at the same time of year.