Fights between male swimming crabs (Liocarcinus depurator) were studied in the laboratory and in the field. These crabs fight readily in the laboratory, interactions being initiated equally often by the larger and the smaller of the two opponents, but usually being won by the larger. Many different combinations of cheliped and swimming leg postures are seen during fights, which fell into 4 categories: 13% involved only stationary display, 38% were resolved by a single approach and retreat by displaying crabs, 9% were resolved by multiple approaches and retreats and 39% involved physical contact between the crabs. Fight length was variable, and depended both on the absolute size and on the relative size of the participants. During the course of the fights, few behavioural differences were observed between the eventual winners and losers. Fights were less common in the field, but the same rules determined their initiation, content and outcome. The results are discussed in the context of game theory.