Résumé: We studied the sexual maturity, batch fecundity and spawning season of the Skipjack Tuna Katsuwonus pelamis. Generalized additive modelling was applied to assess the effect of demographic features on ovary growth. During 2009 and 2010, 673 females were caught on board a purse-seine fishing vessel in the western Indian Ocean. Fish were classified into different reproductive phases and this information was used to determine the size at maturity (L-50), the timing of spawning and the batch fecundity (n = 51). The size at maturity (L-50) for females was estimated to be 39.9 cm. The mean relative batch fecundity was 140 +/- 64 eggs g(-1) of fish. Spawning occurs during the monsoons and inter-monsoon seasons with peaks in the reproductive activity during the North-east and South-west monsoon events. Demographic differences were detected in the reproductive timing of the population and a large investment in gamete production was observed in fish with high hepatic condition and in their first spawning year.