Giesbrecht (1895) was the first to report that the luminescence produced by marine copepods was produced by certain ‘skin glands’. In Pleuromamma abdominalis he described specific sub-cuticular cells which were responsible for the production and secretion of luminous material; such cells always contained a greenish-yellow secretion whilst non-photogenic cells were colourless. The only published histological study of luminous cells in copepods is that by Clarke et al. (1962). These authors reported numerous paired cells in the head of Metridia longa, each pair opening through a common pore, the distribution of which was correlated with areas of fluorescence. Clarke et al. (1962) further showed that the cells in each pair had different staining characteristics, and suggested that each of the two cell types produced a different component of the luminescent system, proposed as ‘luciferin’ and ‘luciferase’ respectively. They hypothe-sized that when stimulated, each cell secreted its contents, generating light as the materials combined in the surrounding seawater. Although there has been much work carried out on the physical characteristics and kinetics of copepod luminescence (e.g. David & Conover, 1961; Clarke et al., 1962; Barnes & Case, 1972; Bityukov & Evstigneev, 1982; Herring, 1983, 1988; Evstigneev, 1983, 1984, 1986; Latz et al., 1988) there is still little known of the comparative distribution of luminous cells in a range of copepod species, and even less is known of their detailed structure.