SUMMARY: Most, or all, wild females of Trichosurus vulpecula bred during the first 6 months of the year and some bred again during the second half of the year. In captive females the oestrous cycle varied in length from 22 to 58 days but all cycles longer than 32 days began between 20 April and 16 June. During two periods of each year, from early February until 20 April and from 16 June until late September, the cycle varied in length from 22 to 32 days (mean: 25·69±0·31 days). The females were polyoestrous and monovular and the gestation period was about 17·5 days. Vaginal smears from mated and non-mated females were alike except that spermatozoa were usually found in smears from mated females on the first or second day after copulation. After oestrus, ovulation and corpus luteum formation occurred alike in mated and non-mated females. There were no differences between corpora lutea of pregnant and non-pregnant females. After parturition oestrus was inhibited by the onset of lactation except in one female which produced two successive young separated in age by approximately the length of one oestrous cycle. Females came into oestrus an average of 8·02±0·18 days after removal of suckling young from the pouch. In pregnant females the gravid uterus was much larger than the non-gravid uterus from 12 days after oestrus until after parturition. The sub-epithelial capillary layer was better developed in the gravid than in the non-gravid uterus during the closing stages of pregnancy. The uterine luteal phase was present 7–8 days after oestrus and began to disappear at about 15 days after oestrus in both pregnant and non-pregnant females. There was no evidence that the luteal phase was longer in pregnant than in non-pregnant females and the occurrence of pregnancy did not extend the normal interval between successive oestrous periods if the young were removed at birth.