Many marine animals that live at depths greater than about 500 m are deeply coloured (generally red, brown, black or purple) even though there is little or no down-welling light in these regions (Clarke & Denton, 1962). The nature and function of many of these pigments are obscure. Partly, no doubt, because of the difficulty of provenance, there have been very few chemical and biochemical investigations on such pigments (see Crozier (1974) for a review of pigments in fishes). We report here an investigation of the pigments of meso- and bathypelagic medusae. The work was initiated following preliminary identification of protoporphyrin in Atolla wyvillei and Periphylla periphylla (Herring 1972): the present work confirms and considerably extends these observations (Head, 1976, M.Phil. Thesis, University of London).