In a survey of five villages in the Eastern Highlands of Papua New Guinea, Serpulina pilosicoli was isolated from rectal swabs from 113 of 496 individuals (22·8%). Colonization rates ranged from 22·6–30·1% in four of the villages but was only 8·6% in the other village. In comparison colonization was demonstrated in only 5 of 54 indigenous people (9·3%) and none of 76 non-indigenous people living in an urban environment in the same region. Colonization did not relate to reported occurrence of diarrhoea, age, sex, or length of time resident in a village. A second set of 94 faecal specimens was collected from 1 village 6 weeks after the first set. S. pilosicoli was isolated from 27 of 29 individuals (93·1%) who were positive on the first sampling and from 7 of 65 individuals (10·8%) who previously were negative. In this case, isolates were significantly more common in watery stools than in normal stools. The annual incidence of infection in the village was calculated as 93·6%, with an average duration of infection of 117 days. S. pilosicoli could not be isolated from any village pig (n=126) despite its confirmed presence in 17 of 50 commercial pigs (34·0%) sampled at a local piggery. Four of 76 village dogs (5·3%) and 1 of 2 village ducks were colonized with S. pilosicoli, suggesting the possibility of cross transmission between humans and animals.