Five aspects of the circadian activity rhythm (wheel running, feeding, drinking, duration of time in, and visits to, the nest box) were studied for 129 days in two species of New Guinean dasyurid marsupials that co-exist in mid-montane semi-mossforest. When housed under controlled laboratory conditions, Phascolosorex dorsalis exhibited a diurnal activity pattern whereas Antechinus habbema was nocturnal. Manipulation of the feeding regimen and reversal of the 12: 12 light-dark cycle demonstrated that the diurnal activity of P. dorsalis was not due to synchronisation to food presentation but reflected the behavioural output from an endogenous circadian pacemaker. P. dorsalis appears to be unique among dasyurids in its diurnality. Results are interpreted in terms of the ecological niche being a division of time as well as space.