Strongyloides stercoralis infections were shown to be aggregated in households in an urban slum community in Dhaka, Bangladesh. Parasitological data on 880 residents living in 280 households were analysed statistically using 3 different tests, each of which yielded significant evidence of household aggregation of S. stercoralis infection. One test was applied to the data after stratification for 4 variables were previously shown to be independently associated with infection. Evidence of household aggregation of infection remained after stratification, suggesting that aggregation is due not only to shared risk factors, but also to either familial genetic predisposition to infection or close contact person to person transmission of infection within households.