Exogenous serotonin has been shown to induce an elevated, flexed posture in crustaceans and has also been hypothesized to enhance aggressive behavior. We conducted three experiments to further investigate the effects of serotonin and serotonin analogs on posture and agonistic behavior in the crayfish Procambarus clarkii. In the first experiment, we recorded behavioral responses to five different concentrations of serotonin injected into the ventral hemolymph sinus. The amine elicited a series of behaviors including the characteristic high, flexed posture, but none were clearly associated with aggression. In our second experiment, we tested serotonin and four serotonin receptor agonists [1-(3-chlorophenyl)piperazine dihydrochloride, 2-methyl-5-hydroxytryptamine maleate, 5-carboxamidotryptamine maleate and alpha-methyl-5-hydroxytryptamine maleate] and measured the ability of each agonist to mimic the actions of the amine. High concentrations of 1-(3-chlorophenyl)piperazine dihydrochloride most closely mimicked the actions of serotonin; 5-carboxamidotryptamine maleate induced a high stance, but did not otherwise induce effects similar to serotonin. In our third experiment, we conducted an analysis of fighting behavior between pairs of crayfish that had received injections of control saline, serotonin, or 5-carboxamidotryptamine maleate. Serotonin generally reduced the level of aggression between opponents, whereas 5-carboxamidotryptamine maleate enhanced the performance of several agonistic behaviors.