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Natricine colubrid snakes, including the grass snake, Natrix natrix, are frequently involved in complex social behaviour during the reproductive season. During these social behaviours, several males may simultaneously court a single female, resulting in a ‘ball’of mating snakes in which each male ‘combats’with rival males by ‘tail wrestling’(see Madsen & Shine, 1993). I performed some experiments in outdoor enclosures for testing the male‐male competition and the determinants of mating success in male grass snakes involved in such ‘ball’aggregations. I demonstrated that competition between males occurred both when a single female was available to multiple males and when two females were simultaneously available to males. The larger males achieved more copulations than the smaller ones, thus demonstrating that body size is a crucial determinant of the individual mating success. It was not clear which …