The effect of territorial behaviour on the density and stability of Tawny owl (Strix aluco(L.)) populations was studied in three habitats during 1971–74: deciduous woodland (Wytham, near Oxford), mixed farmland (south Warwickshire) and conifer plantations (Dumfries). Territory size was determined by plotting the position of territorial challenges (hoots) of males; these were individually recognizable.In all three areas, pairs of owls maintained contiguous territories which changed little in size and configuration from year to year. In Wytham, this was in spite of wide fluctuations in the abundance of rodent prey. Territory size was most variable on farmland (17–74 ha) where it was inversely correlated with the area of closed woodland contained within territories. The mean size of territories was significantly smaller in the deciduous woodland (18.2 ha) than the coniferous woodland (46.1 ha) or farmland (37.4 ha). This variation was probably ultimately related to the availability of prey in different habitats.In Wytham, the annual rate of disappearance of territory‐holding males was around 15% and, except after a poor breeding season, birds which disappeared or were removed from their territories were replaced rapidly. Territory boundaries remained the same after replacement and the number and spatial configuration of territories (around 30) was similar to that found 14 years earlier. There is some evidence that individual pairs would be able to raise more young if they could defend larger territories than is possible at the current population size.Recoveries of birds ringed nationally indicate that owls survive less well without territories, and that young suffer heavy mortality in their first autumn. The proportion of young recovered was highest in years when many were ringed. Examination of 276 corpses (mostly road casualties) suggests that many young starve soon after becoming independent.These observations support the idea that the stability of Tawny owl populations results from the territorial behaviour of residents acting to produce subsequent mortality among birds unable to establish their own territories. This regulation probably applies over wide areas and a variety of hab