44 albino mice of SWISS strain, isolated for 21 days, 25 in the usual plexiglas laboratory cages, 19 in large complex boxes, were tested for aggressive behaviour both in a neutral arena and in their own territory. While mice isolated in usual conditions are similarly aggressive in their territory and in a novel area, the mice isolated in a large box are strongly aggressive in defending their territory and only mildly aggressive in a novel area. The hypothesis that a too restricted environment inhibits territory formation in mice is discussed.