This study discusses occurrence and biosynthesis of melanins and indigoids, pigments with indole structure produced by Mollusca. Indigotin and 6,6'-dibromoindigotin (DBI), constituents of mollusc purple, are treated in some detail. Their colourless precursors in the organism are presumed to be of dietary origin. The hypothesis is formulated that formation of DBI is due to a detoxification mechanism; it is suggested that purple-producing species have developed it into an enzymatically controlled defensive system against large predators. This first reaction step is then followed by spontaneously proceeding reactions involving oxygen and, for DBI, light, leading to the formation of the coloured pigment, which seems to be of no further use to the animal. Experiments with purple from three Mediterranean species are reported.