The timing of reproduction was investigated and compared in natural pop ulations and laboratory colonies of three species of dasyurid marsupials, Antechinus stuartii, A. swainsonii and A. flavipes. In the natural populations, reproductive events occurred only during the austral winter (July-September), and began at almost exactly the same time each year. The mating period lasted–3 weeks, and coincided with the onset of pouch changes (oestrus) in females. In males, spermatozoa appeared in the urine up to five weeks before the first matings occurred, but sexual activity tended to be suppressed during this time by female aggression. In the laboratory colonies, most individuals were paired (females with males) and housed under 12L: 12D or ambient light. Irrespective of the light regime, all first-year animals entered reproductive condition at the same time or only slightly earlier than those in the natural populations. However, in females housed for two years under 12L: 12D, oestrus occurred–5 weeks earlier, on average, than in field individuals, and–5 weeks earlier than in equivalent individuals housed under ambient light. Males again achieved reproductive condition before females, and may have stimulated and helped to synchronize oestrus. These findings showed that the timing of reproduction is controlled by an endogenous circannual rhythm, which is probably entrained by some component of the photoperiod. The timing mechanism ensures that all young are nursed and weaned in spring and summer when food is most predictably abundant, and hence maximizes individual reproductive success.