Female wasps of the primitively eusocial species Ropalidia marginata may be classified, by a statistical analysis of their time-activity budgets, into three behavioural castes namely Sitters, Fighters and Foragers. We show that Foragers are primarily responsible for the risky task of foraging for food and have very poorly developed ovaries. Sitters and Fighters forage rarely if at all but share the bulk of the intra-nidal tasks such as feeding larvae and building the nest. Both Sitters and Fighters have better developed ovaries than Foragers. Queens of most colonies belong to the Sitter caste. There are no obvious morphological differences between queens and workers or between the behavioural castes. Queens are not necessarily the most dominant individuals in their colonies. Instead, most dominance behaviour is performed by a group of workers (the Fighters). Division of labour and social organization are achieved through behavioural caste differentiation and not, as in many other species studied, through a dominance hierarchy led by a despotic queen suppressing all her nestmates into worker roles. This suggests that behaviour patterns in such primitively eusocial insects are likely to be moulded by a complex interaction between selection at the individual and colony levels.