Gonadal development in immature, pubertal, young mature, and sexually mature male common dolphins (Delphinus delphis) was assessed for 212 dolphins collected between 1991 and 2003. The sample consisted of animals stranded along the Irish and French coasts, and also bycatch samples from Irish and French observer programs. Through histological analysis of the testes, individuals were categorized into reproductive stages by using characteristics of their gonadal morphology. These stages were immature, pubescent, young mature, and sexually mature. Male common dolphins were 102–233 cm in length and 0–28 years of age. Sexually mature individuals were 195–233 cm in length and 8–28 years of age, and the average age at attainment of sexual maturity was 11.86 years. Combined testes weight for mature male dolphins ranged from 0.45 to 5 kg, which is relatively large considering the overall size of the dolphin. Reproductive seasonality was found to occur, as evidenced by marked seasonal changes in both testes weights and cellular activity in testes outside the mating period; the mating period was estimated to take place during May–September. Moderate sexual dimorphism and large testes suggest sperm competition and a promiscuous mating system, with female common dolphins mating with multiple mates.