Aggressive interactions are essential for resource distribution and population dynamics of many animal species. Aggression can also help invasive species to wrestle the resources from other species and invade new habitat. To examine the effects of intra-species aggression on range expansion, we compared aggression levels of signal crayfish (Pacifastacus leniusculus) at the core and front of the invasion range in recently invaded regions of Croatia. More pronounced aggression was observed in core populations with high population densities, indicating potentially greater importance of highly aggressive behaviour in populations with higher competitive pressure. Despite better overall physical condition, individuals at the invasion front generally displayed lower levels of aggression and regularly lost interactions to individuals from the invasion core. Rather than providing a competitive advantage during range expansion, aggression may be more expressed in established populations, priming the individuals for future expansions while also driving the dispersal outward. The observed difference in aggression along the invasion pathway demonstrates that traits that help individuals to overcome challenges of their environment, such as competition against conspecifics, can drive the invasion dynamics of a successful invader in a new environment in terms of both niche competition and intrinsic expansion dynamics.