A comparative study of the following luminous copepods was undertaken: Metridia lucens, M. longa, M. princeps, Pleuromamma robusta, P. xiphias, Heterorhabdus norvegicus, H. robustus, Heterostylites longicornis, Lucicutia grandis, Hemirhabdus grimaldii, Disseta palumboi, Euaugaptilus magnus and Centraugaptilus horridus. Flashes produced by electrical stimulation (a.c. or condenser shocks) and mechanical stimulation were recorded photoelectrically. Flashes lasted from 2 to 37 sec. Latencies of some species (Metridiidae), following electrical stimulation, were very short, 7–9 msec. Intensities ranged from 0·02 × 10–5 to 14·4 × 10–5μW/s cm2 of receptor surface at 15 cm distance (0.0045 × 10–2 to 3·24 × 10–2μW/Cm2 at 1 cm) (10–20°C). Luminous glands of Metridiidae, Lucicutiidae and Augaptilidae are autofluorescent; the location of the luminous glands in these families and in Heterorhabdidae is described. Two kinds of glandular cells (types 1 and 2) occur in the luminescent areas. The cells are large saccular structures containing granular or homogeneous material, and are distinguished by staining peculiarities. Cell types 1 and 2 open through common pores, and may be the source of luminous reactants. Some measurements of luminescence in other pelagic animals are presented, for comparison with copepods, viz. Aequorea macrodactyla, Aeginura grimaldii (Hydromedusae), Periphylla periphylla (Scyphomedusa), Meganyctiphanes norvegica and Acanthephyra pelagica (Crustacea), Myctophum punctatum (Teleostei). Our present knowledge regarding luminescence among copepods is reviewed.