We tested the hypothesis that agonistic behavior and interference competition induce species replacements in freshwater decapods. Our model organisms were two crayfish (the indigenous Austropotamobius italicus and the non-indigenous and invasive Procambarus clarkii) and the indigenous river crab Potamon fluviatile. A first experiment was aimed at analyzing the agonistic behavior of similarly-sized males in pairs of the three species combinations. Records were taken for an hour per day during five consecutive days of combats. Results showed that P. fluviatile was dominant over the two crayfish species and P. clarkii over A. italicus, as confirmed by their field distribution. In nature, the non-indigenous species might even reach higher levels of dominance over A. italicus. In fact, both the larger body size and the 'stronger' chelae of P. clarkii can induce asymmetries in fighting ability. Pairs composed of the non-indigenous crayfish and either A. italicus or P. fluviatile did not form stable hierarchies, possibly due to the failure of status recognition. In a second set of experiments, the agonistic behavior of the three species combinations was studied in the presence of either food (earthworms) or an artificial shelter (a 10 cm-long piece of a PVC pipe). As expected, resources influenced fighting and dominance translated into a differential capability to compete. In a third experiment, in which we measured shelter use by each species in a non-competitive context, shelters were more extensively occupied by A. italicus (which is dependent on natural crevices as hiding places) than by the other two species (which usually dig burrows). We expected that Potamon and Procambarus would gain less from occupying the offered shelter than Austropotamobius, and therefore their defense should be less vigorous. To the contrary, the presence of a rival strengthened the attraction to the shelter of these two species, in particular when river crabs were opposed to A. italicus.