The ecology of reproduction was studied in the snake Natrix maura. The presence of sperm in cloacal fluid was used to show the size at maturity, the season of mating and, together with information on the persistence of sperm in cloacal fluid of females after mating, the frequency of mating. These results suggest that each female mated several times per year. Female N. maura grew substantially after maturity, and clutch size increased with body size. Current and expected future reproduction increased in parallel during the period of growth after maturity; subsequently they were constant. This pattern would be expected to lead to a constant proportional allocation of energy to current reproduction. Reproductive effort (eggs/body energy) was independent of age as measured by growth rings. There was some evidence that not all stored energy was used in reproduction in the current season even when some follicles regressed.