Lucerne labelled with 131I or 125I was used to mark field colonies of the harvester termite Hodotermes mossambicus (Hagen) in South Africa. The foraging area was delineated by plotting the distribution range of radioactive workers collected on the soil surface over a period of up to 57 days. Foraging areas of single colonies varied between 0.5 and 3.1 ha. These areas were not necessarily the exclusive territory of a single colony. Inter-worker aggression assays confirmed these conclusions.