A random survey of 680 dwellings was undertaken to determine the prevalence and mode of transmission of Giardia intestinalis in Mount Isa. Of the population, 4.5% were shown to have cysts in their stools; the highest prevalence rate of 12.0% occurred in children aged one to five years. Epidemiological studies strongly suggest that the disease, which is endemic in the city, is being transmitted by hand-to-mouth contamination; schools, preschools, kindergartens and play groups are important foci of transmission. No evidence was found that the parasite was being brought into the city from outside, or that people were acquiring it from water or domestic pets. Symptoms consistent with giardiasis were more commonly present in children than in adults. Approximately two-thirds of infected persons were asymptomatic. The diagnosis of giardiasis as based on symptoms alone was inadequate, although more likely to be correct in children than in adults.