Temporal distributions of matings and births were studied in a feral population of Bizet sheep (Ovis aries) introduced to Ile Longue in the subantarctic Kerguelen Archipelago. Two rutting peaks occurred each year, in summer and winter. Most births occurred in winter, a period that was not favorable for the survival of young. Lambs born in midsummer also were born too late with regard to the most favorable period in spring. Individual ewes did not consistently breed in 1 season but switched from summer to winter breeding. Regardless of whether they were lactating or nonlactating in summer, almost 70% of females gave birth the subsequent winter. Only 25% of females observed in winter (lactating or nonlactating) also lactated in summer. Differential fertility and reproductive success may be due to seasonal variation in body mass. Females were heavier in summer than winter. The biannual reproductive cycle also may result from the early reproductive history of ewes. Sexual maturity occurred earlier for the winter compared with the summer cohort. Therefore, most females of both cohorts gave birth for the first time in winter. Our results reveal that feral domestic animals will not always quickly adapt to different environmental conditions.