Abstract. Realistic predictions from game theory about how animals should behave in a variety of competitive situations require knowledge of the costs associated with alternative strategies. In this study, the energetic cost of agonistic behaviour in velvet swimming crabs was investigated. There was a linear relationship between the rate of oxygen consumption and the rate of beating of the ventilatory pumps (the scaphognathites) and there was no evidence of significant anaerobic energy production during agonistic interactions. Energy expenditure by the crabs should therefore have been closely related to their oxygen consumption and scaphognathite activity. An irregular pattern of scaphognathite activity was recorded during agonistic behaviour, incorporating periods of extreme hyperventilation as well as cessation of scaphognathite activity. Maximum scaphognathite rates were among the highest reported for a range of similar decapod crustaceans. During agonistic behaviour, the respiratory activity of the interactants was related both to the degree of escalation and to the duration of the interaction. Recovery was prolonged in some cases. The estimate of total energetic cost included respiratory activity during the recovery phase and was correlated with interaction content and duration for losers only. However, there was no significant difference between the estimates of energy expenditure of winners and losers. As injuries are rare, energy expenditure may account for a large proportion of the total cost of contest behaviour in this crab.