A one-year study of gastro-intestinal parasitism in a free-ranging population of maras at Whipsnade Wild Animal Park, UK, revealed a strong relationship between membership of social units and both intensity and prevalence of infection. The mara, a hystricomorph rodent from southern Argentina, has a social organization including both monogamy and communal denning of the young, an apparently unique combination among mammals. From October 1992 to September 1993, strongyloid parasite loads were estimated from faecal egg counts. A minimum adequate model was fitted to the data using the Genstat statistical package. This showed that family membership had a highly significant effect on the intensity of egg shedding in faeces, and a significant effect on the prevalence of infection. After controlling for both extrinsic environmental and intrinsic demographic factors, homogeneity of infection was greater within than between families and adult pairs.