We analysed seasonal variation in the size and composition of groups, and individual behaviour, of a wild population of Greater Rheas (Rhea americana) in the Argentinean pampas. We performed seasonal censuses over 8 years, and made focal observations on 59 individuals from different groups to assess the proportion of time spent in vigilance, feeding, walking, preening or resting, and noted the number of courtship displays, copulations and aggressive interactions. Rheas were solitary or formed groups of 2–20 individuals. Mean typical group-size (TGS) over the year was 7.61 birds but varied significantly with season, TGS decreasing during the breeding season (spring), possibly owing to formation of harems. TGS of males decreased in spring whereas TGS of females did not vary seasonally. During spring, individuals spent more time vigilant and less time feeding, and were involved in many courtship interactions. Most aggression (72%) was between males and mainly occurred during winter and spring, suggesting strong intrasexual competition. Social organisation and group dynamics appear to be profoundly affected by the mating system of the Greater Rhea.