Using two populations of Aegla araucaniensis, one syntopic with two congeneric species (A. abtao and A. denticulata) and the other no, we assessed agonistic behavioural variations associated with interspecific competition (type of interaction, intensity of aggression, and frequency of dominance in two-and three-species encounters). In general, there was a low level of aggression among interacting individuals. The frequency of agonistic responses tended significantly towards more aggressive behaviours in A. araucaniensis that are not naturally exposed to interspecific competition compared with those that are. The competitive hierarchy was dominated by A. abtao, followed by both populations of A. araucaniensis. Aegla denticulata was always the subordinate species. We can conclude that: (1) the dominance hierarchy is coherent with the distribution patterns of the species in their natural habitat; (2) unlike other species of freshwater decapods, the nature of the studied aeglids is fundamentally passive; (3) in A. araucaniensis, interspecific competition can generate changes in the expression of behavioural traits; (4) in decapod crustaceans, the hierarchies of dominance may be established by conditions other than fighting abilities and/or intensity of agonistic interactions.