Ridgeia piscesae from a hydrothermal vent and lucinid and thyasirid bivalves from inshore Canadian and UK waters, known to contain sulphur-oxidizing symbiotic bacteria, had lipids rich in 16:0,16:l(n-7) and 18:l(n-7) fatty acids in both bacteria-rich trophosome or gill tissue and in tissues without symbiotic bacteria. The results are consistent with the animals deriving these fatty acids from their sulphur-oxidizing symbionts. Ridgeia piscesae, Lucinoma annulata, Parvilucina tenuisculpta, Lucinoma borealis and Myrtea spinifera also contained substantial amounts of the non-methylene-interrupted dienoic fatty acids 20:2δ5,13 and 22:2δ7,15. It is proposed that these fatty acids are produced by chain elongation and δ5 desaturation in animal tissues of 18:l(n-7) produced by the bacterial symbionts. Thyasira flexuosa did not contain 20:2δ5,13 or 22:2δ7,15 but instead contained 18:l(n-ll) and 20:l(n-13) which were not present in the other species analysed. It is proposed that 18:l(n-ll) and 20:l(n-13) arise from the δ9 desaturation of 20:0 and 22:0, respectively, followed by chain shortening of the mono-unsaturated fatty acid products of δ9 desaturation. It is considered that 20:2δ5,13 and 22:2δ7,15 are formed in the animals in response to a relative excess of 16:0, 16:l(n-7) and 18:l(n-7), accompanied by a relative deficiency of (n-3) and (n-6) polyunsaturated fatty acids. The results are discussed in relation to the lipid nutrition of marine invertebrates containing bacterial symbionts.