Establishing accurate demographic information for free-ranging populations of animals is difficult without knowledge of individual chronological age. We estimated the birth dates of 743 koala (Phascolarctos cinereus) joeys at 3 sites in Queensland, Australia, using body mass obtained from a reference population with known birth dates. From these age estimates we compared the annual distribution of births across calendar months. At all 3 locations about 60% of births occurred between December and March. The annual pattern of births was identical for males and females within locations, but overall annual patterns of births differed between the southern and northern sites. We conclude that koalas can bear offspring in every month of the year, but breed seasonally across Australia, and that a sex bias in the timing of births is absent from most regions.