We investigated timing of reproduction in a wild population of northern brown bandicoots (Isoodon macrourus) in the Australian Wet Tropics. Almost all births occurred during the late dry season and early wet season, and most adult females (78–96%) were carrying pouch young during those times. Litter sizes ranged from 1 to 6 pouch young (mean = 3.1) and was not influenced by season. Adult males had significantly larger testes in the late dry and early wet seasons, corresponding with the peak in births. Daylength was the only environmental factor that predicted the presence of a litter; when daylength exceeded 12 h, more than 70% of captured females were carrying pouch young, and most (94%) births were estimated to have occurred on days with >12 h of daylight. Various environmental factors have been proposed as a cue for breeding in I. macrourus, with daylength though to be the primary cue initiating breeding in temperate Australia, but temperature and rainfall thought to be more important in the tropics. Our data suggest that in the Australian Wet Tropics, increasing daylength in the late dry season acts as the primary cue for breeding.