The salmon louse (Lepeophtheirus salmonis) is a major parasite of salmonid fish in the marine environment. The interaction between the parasite and the host upon infection is not completely understood. However, it is clear that the parasite influences the host and its immune system. Prostaglandins produced by parasites such as flatworms, roundworms and ticks are documented or assumed to play a role in immunomodulation of the host. In the salmon louse, the effect of prostaglandins on the host is assumed, but remains to be documented. In this study, a salmon louse prostaglandin E2 synthase (LsPGES2) is characterized. Ontogenetic analysis showed that LsPGES2 is relatively stable expressed during development. The highest level of expression was seen in the free living stages, although elevated levels of LsPGES2 were also found in adult females. In copepodids, the LsPGES2 is found around muscle cells, while it is observed in the reproductive organs of adult female lice. LsPGES2 expression was knocked-down by RNA interference in nauplii, but emerging copepodids did not display any changes in morphology nor ability to infect and develop to adult stages on fish. Additional knock-down of LsPGES2 in adult female lice did not produce any characteristic changes in phenotype nor reproductive output. It is concluded that under these experimental conditions, knock-down of LsPGES2 did not affect any essential functions of the salmon louse, neither in the free-living nor the parasitic stages.