ABSTRACT.A random survey of children under 11 years was undertaken to determine the prevalence ofGiardia intestinalisin two areas of the City of Logan, South East Queensland. Parasitological examination of single stool samples showed that in Shailer Park, where 81% of the houses have septic tanks, the prevalence rate was 5.7%, with 9.8% of households having children passing cysts. At Rochedale where 98% of the houses are connected to the mains sewerage the corresponding figures were 2.0% and 3.1%. Most if not all the infections were acquired locally. No correlations were found between the prevalence ofG. intestinalisand the sex or age of the children, symptoms of diarrhoea or socioeconomic conditions. Clustering of cases was demonstrated in one part of Shailer Park but no point source of infection was demonstrated. One possible explanation for the higher prevalence rate at Shailer Park is that children become infected with cysts found in run‐off from septic effluents which accumulate in gardens and gutter