Symphurine tonguefishes (Pleuronectiformes: Cynoglossidae) have been found at hydrothermal vents on several submarine volcanoes in the western Pacific Ocean, often in great numbers. Marine fish require dietary polyunsaturated fatty acids (PUFA) for growth and development, but it is unclear how this need is met in environments based on chemosynthetic production. Furthermore, the trophic ecology and nutritional requirements of symphurine tonguefishes at vents are generally unknown. To address these knowledge gaps, fatty acid composition was determined in muscle, liver, gonad and eyes of Symphurus thermophilus (Mariana Arc; 21-23°N, 142-144°E) and Symphurus sp. A (Tonga Arc; 21º09ʹ S, 175°45ʹ W) sampled in October 2005 and May 2007, respectively. All tissues of Symphurus spp. contained substantial levels of PUFA. Relative amounts of most PUFA in Symphurus spp. muscle were similar to levels measured in non-vent demersal fish from temperate waters worldwide. Principal components analysis (PCA) using Symphurus sp. A (Tonga Arc) fatty acid profiles revealed three distinct clusters of samples that were related to tissue type. Similar trends could not be discerned for S. thermophilus (Mariana Arc). The fatty acid composition of Symphurus spp. suggests that PUFA-rich prey were not limiting at the sampled vents, although we lack information about how these fish metabolize lipid. Stable carbon isotope ratios (δ13C) of several individual fatty acids of fish from Nikko and Daikoku suggest a photic zone origin. Benthic-pelagic coupling on intra-oceanic volcanic arcs may strongly influence the presence and persistence of resident vertebrates.