Mantis shrimp are benthic, predatory marine crustaceans that have complex agonistic interactions. These crustaceans are divided into two functional groups based on the morphology and use of their raptorial appendage: smashers and spearers. Most research on the agonistic behaviours of mantis shrimp has focused on smasher species and on contests between asymmetrical and same-sex individuals. No studies have investigated the intersexual and intrasexual interactions of size-matched spearer individuals. I conducted a laboratory experiment using Squilla empusa, a spearer that lives in the Gulf of Mexico, to determine if agonistic differences exist between males and females. The results suggest that (1) although threat displays are rare in both males and females, male aggressive interactions involving physical contact are common, (2) males engage in more aggressive behaviours and interactions than do females, (3) females are less aggressive toward both males and females than males are toward males and females; interactions involving females are usually passive, non-striking, whereas interactions involving males can result equally in a strike or passive behaviour, (4) males are more aggressive than females, and (5) an increase in the number of treatment individuals resulted in an increase in the number of interactions. The behaviours of Squilla empusa are compared with literature reports concerning other species of mantis shrimp. The differences in habitat, feeding method, vision, and burrow type may explain the differences between smashers and spearer agonistic behaviours.