We compared aggressive behavior among four crayfish species by measuring the number and duration of agonistic acts occurring in fighting bouts between intraspecific pairs of animals. Our study included 3 congeneric species from the family Cambaridae (Orconectes rusticus, Orconectes propinquus, and Orconectes immunis) and 1 from the family Astacidae (Pacifastacus leniusculus). Based on our measurements, crayfishes differed in their level of aggressiveness and differed in the extent to which they used particular agonistic behaviors. Contrary to previous reports, O. rusticus did not appear to be especially aggressive. Of the species in our sample, O. rusticus was clearly more aggressive than only 1 species, O. immunis. Orconectes propinquus was likewise significantly more aggressive than O. immunis. Pacifastacus leniusculus (Astacidae) appeared to be the most aggressive, differing from the other 3 species in the amount of time spent fighting and in individual behaviors, especially those associated with use of antennae. Understanding species-specific differences in aggressiveness may yield insights into species replacements in natural crayfish populations, and may contribute to the value of these animals as models for investigating the neural basis of aggression.