The phenomenon of acid secretion has arisen independently many times, for feeding, for penetrating calcareous material to make a shelter or to pursue prey, or for defence. This last category contains examples from many groups of marine organisms, in which acidic allomones may serve to deter fouling by epibionts or predation by teleost fish. Histochemical and histophysical methods have increased the precision with which acidic allomones can be identified within acid vacuoles, cells, glands, sacs or capsules. Acidic allomones are produced in pleurobranch molluscs by unicellular epidermal glands, by multicellular subepidermal sacs, or by an arborescent internal acid gland, connected to the buccal mass by a thick medio-dorsal duct. Some species have all three types. In the case of the epidermis, discharge is probably triggered directly by the trauma of attack by a predator (effectively, holocrine discharge), but the other types of gland are discharged by contraction of a strong muscle sheath, without tissue damage. Cypraeid molluscs have extensive, mobile mantle lobes which can ensheath the massive shell temporarily. The epidermal acid glands are orientated within the mantle so as to discharge away from the shell. The lamellariid and philinid molluscs have internal shells, well protected from chemical attack, and the multicellular, subepidermal acid glands are situated over all the surfaces of the body. In the tunicates investigated, both blood and tunic were neutral in Ciona intestinalis, but they were both acid (pH 1) in Ascidiella aspersa. In Ascidiella scabra and Ascidia mentula the blood was neutral, and the tunic alone was acid. The acid tunics contained abundant blind lacunae. It is unlikely that these acid-filled lacunae correspond to the blind-ending arterial ampullae described by other workers. Discharge is probably brought about directly through the trauma of attack. The perennial species of the alga Desmarestia, D. aculeata, exhibited a pH near to neutrality. Two annual species, D. dresnayi and D. ligulata, contained acid of pH 1–2, and showed strong precipitation of BaSO4 after BaCl2 treatment. Sections confirmed that the acid is normally held within the swollen parenchymatous cells which pack the sporophyte frond. The cuticle is comparatively thin in the acidic species. In conclusion, it is argued that acid secretion may be the allelochemical analogue of warning or aposematic coloration.