Summary.It has been shown by rearing and breeding experiments that the amphipod Jaesa falcata (Montagu 1808) is a polymorphic species.The species falls into two main classes or divisions, characterized particularly by the differing setation and shape of the aecond antennae, and the second gnathopods of the males. These divisions are called here, in accordance with their appearance, the Broad end the Narrow Forms. There is a third Form, in which the antennal oharacters of both the other Forms ere combined with either the Broad gnathopod or the Narrow one.In addition a number of Minor variants occur within the limits of the two main divisions.The species also shows male intersexuality.The trivial names enumerated below we consider synonyms of Jassa falcata (Montagu). It is not necessary to give the full synonymy list here as that is included in the following review of the bibliography.