Fragmentation of habitat has been found to create dispersal barriers for some species. Less well understood is the possibility that physical barriers to dispersal between habitat patches might be reinforced or modified by behavioral barriers. We assessed whether there are differences in aggressive behavior between Peromyscus leucopus from large continuous tracts of woods and habitat islands (i.e., isolated woodlots). Data from studies of mainland and true island habitats suggest that mice are less aggressive on islands. If decreased aggressiveness also characterizes mice in habitat islands, populations may be more permeable to immigrants than their physical isolation suggests. To determine if this pattern is evident in habitat islands, we paired adult males from the two habitats (continuous woods and isolated woodlot) in a neutral arena and measured frequencies of aggressive behaviors. Males from both habitats were equally aggressive. We suggest that physical isolation of habitat islands is not ameliorated by decreased aggression to immigrants as it seems to be in the mainland/island complex.