AbstractIn terrestrial plethodontid salamanders, aggressive behaviour is thought to function in the spacing of territorial residents among contested cover objects on the forest floor. Such behaviour, when exhibited toward heterospecifics, plays an important role in the competitive interactions between species. We compared levels of aggressive behaviour in intra- and interspecific contexts in two species of sympatric salamanders (Plethodon ouachitae and P. albagula) that have similar ecological requirements but differ in adult size. We also tested the effectiveness of such behaviour in holding cover objects (territorial foci) in the laboratory and on the forest floor. We predicted that if one species were more aggressive than the other, then that species would have greater success in obtaining and holding cover objects. In laboratory trials, residents of P. ouachitae (the smaller species) were extremely aggressive in both intra- and interspecific contexts. Individuals of P ouachitae delivered bites at a rate 14 times that of previously studied species of Plethodon and were significantly more likely to escalate to biting when tested as territorial residents (in intra- and interspecific trials) and as intruders (in interspecific trials). Plethodon albagula exhibited a lower level of aggression, similar to other species of Plethodon. In laboratory trials, in which salamanders competed for cover objects of differing quality, residents of P. ouachitae were effective in expelling conspecific intruders, and they were marginally effective at expelling intruding P. albagula. Residents of P. albagula were less effective in expelling conspecific intruders and did not expel intruding P. ouachitae. We conclude that the extreme aggression exhibited by P. ouachitae enabled it to expel intruders from artificial cover objects and to invade cover objects held by larger heterospecific residents. Field data supported intraspecific defence of cover objects by P. ouachitae, but results for P. albagula were inconclusive. These results are consistent with the geographic distributions of these species (P. ouachitae typically outnumbers P. albagula in the Ouachita Mountains) and provide an example of a behavioural mechanism overcoming a size-related disadvantage.