The nature ol the agonistic interactions between males, competing for females or space, are compared with those between females in the spiderLycosa tarentula fasciiventris. Sex differences are found in the behavioural repertoire, the duration and the outcome of the contests. Interactions between males are shorter, less intense and less variable than those between females. The duration of male agonistic interactions does not decrease as size difference increases. Prior access to the female terrarium consistently predicts the outcome of male agonistic interactions, but size difference does not. Sex differences are interpreted in relation to the optimization of the cost-benefit balance by individuals during interactions.