The production of young by three groups of eight possums, housed in a natural, a short-day (10 h light:14 h dark) and a long-day (14 h light:10 h dark) photoperiod was monitored for 24 months to determine the role of photoperiod on the occurrence and duration of the breeding season. Possums were housed in each of the three light regimens on 22 November. The possum has a gestation duration of 17.5 days. Removal of the newborn young leads to ovulation approximately 9 days later and birth 26 days later. Repeated removal of pouch young was used to determine the duration of the breeding season. Possums in a natural photoperiod showed two breeding seasons for the 2 year observation, from March to October of each year. Over the same 2 year period, there were three breeding periods in possums in short-day and long-day photoperiod. A comparison of the timing of the first breeding season showed that short-day photoperiod brought the breeding season forward by 2 months and long-day photoperiod delayed it by 2.5 months. The total number of months in which births occurred was the same for natural and short-day photoperiod (16 out of 24 months) and less for long-day photoperiod (11 months). Fertility in the three groups was also examined. Within each group, there was no change in fertility with consecutive seasons.(ABSTRACT TRUNCATED AT 250 WORDS)