The development of Euterpina acutifrons is described from observations of individuals reared in the laboratory. The characteristics which enable the various stages to be identified have been listed and the feeding mechanism of the larvae described. Differences from the usual developmental pattern of the Harpacticoida are noted and it is suggested that these are related to the pelagic habit of Euterpina. Dimorphism in the male is described and the course of development of the dimorphs is shown to differ from schemes previously suggested for other copepods. Dimorphism, which is manifested at the moult of the third copepodid, is shown to be independent of temperature.