Lepeophtheirus salmonis is an ectoparasitic copepod that causes serious disease outbreaks in both wild and farmed salmonids. As the relationship between L. salmonis and its hosts is not well understood, the current investigation was undertaken to investigate whether any immunomodulatory compounds could be identified from secretions of L. salmonis. By incubating live L. salmonis adults with the neurotransmitter dopamine in seawater, we were able to obtain secretions from the parasite. These were analyzed by RP-HPLC column, as well as LC–MS. L. salmonis secretions contained a compound with the same retention time and mass of PGE2. The identity of this compound as PGE2 was confirmed by MS—in source dissociation. The concentrations of PGE2 in L. salmonis secretions ranged from 0.2 to 12.3 ng/individual and varied with incubation temperature and time kept off the host. Prostaglandin E2 is a potent vasodilator and thought to aid in parasite evasion from host immune responses. This is the first reported evidence of prostaglandin production in parasitic copepod secretions and its implications for the host–parasite relationship are discussed.