The neuromodulator serotonin has widespread effects in the nervous systems of many animals, often influencing aggression;and dominance status. In crayfish, the effect of serotonin on the neural circuit for tailflip escape behavior was found to depend on the animal’s social experience. Serotonin reversibly enhanced the response to sensory stimuli of the lateral giant (LG) tailflip command neuron in socially dominant crayfish, reversibly inhibited it in subordinate animals, and persistently enhanced it in socially isolated crayfish, Serotonin receptor agonists had opposing effects : A vertebrate serotonin type 1 receptor agonist inhibited the LG neurons in dominant and subordinate crayfish and had no effect in isolates, whereas a vertebrate serotonin type 2 receptor agonist enhanced the LG neurons’ responses in all three types of crayfish. The LG neurons appear to have at ]east two populations of serotonin receptors that differ in efficacy in dominant, subordinate, and socially isolate crayfish.