Hyaluronan (HA) has been identified as the principal glycosaminoglycan (CAG) in the highly hydrated, extracellular body matrix of the larval stage (leptocephalus) of seven species of true eels (Teleostei: Elopomorpha: Anguilliformes) and the ladyfish Elops saurus (Elopiformes), and was found as a minor GAG component in the bonefish Albula sp. (Albuliformes). Identification was based on: (1) HPLC separation of unsaturated disaccharides derived from chondroitinase ABC digests of whole-body GAG extracts; (2) 1H NMR analyses of native GAG polymers; and (3) degradation of GAG extracts by Streptomyces hyaluronan lyase. The unsaturated disaccharide 2-acetamido-2-deoxy-3-O-(beta-D-gluco-4-enepyranosyluronic acid)-D-glucose (DeltaDi-HA) accounted for 92.4-99.8% of the total disaccharides in chondroitinase digests. Trace amounts of unsaturated disaccharides of chondroitin sulfate were also present. Two-dimensional gCOSY spectra of the native HA polymer were similar for all species. Proton assignments for the HA disaccharide repeat (GlcAbeta1-3GlcNAcbeta1-4) in D(2)O, based on gCOSY, DQF-COSY and TOCSY analyses for the eel Ahlia egmontis, were concordant with published chemical shifts for HA oligosaccharides. In addition to its presumed role in maintaining the structural integrity and hydration of the gelatinous body of the leptocephalus, HA is postulated to function as a storage polysaccharide in those species in which it is the predominant GAG.