Ecological factors affecting the diel activity of rodents in a sedgeland habitat within Białowieża Primeval Forest were examined by live trapping. The results of a short-term but intensive trapping scheme, with checks every 3 hours, confirmed known differences in activity patterns between voles and mice. We observed an even distribution of vole activity around the 24-h cycle, rather than a marked preference for nocturnal activity in summer. Ten-year data from trapping of the same rodent assemblage dominated by Microtus oeconomus showed that an increase in population density of the dominant species resulted in more diurnal activity of those voles and the co-occurring bank voles. This shift of activity seemed to result from social tensions in a crowded habitat that mostly affected young root voles, as well as individuals of subordinated species. On the other hand, increased predation pressure exerted by weasels Mustela nivalis encouraged more nocturnal activity of voles. The shift was either a direct response of local prey to increased activity of the local diurnal predator, or the ancient anti-predatory response of mammals seeking safety in darkness.