Structural characterization of fucosylated chondroitin sulfates from sea cucumbers Apostichopus japonicus and Actinopyga mauritiana. Two samples of fucosylated chondroitin sulfate (FCS), AJ and AM, were isolated from holothurian species Apostichopus japonicus and Actinopyga mauritiana, respectively. Purification of FCS was performed by ion exchange chromatography followed by gel filtration. Structure of the biopolymers was elucidated using chemical and NMR spectroscopic methods. Both polysaccharides were shown to contain a typical chondroitin core built up of repeating disaccharide units →3)-β-d-GalNAc-(1→4)-β-d-GlcA-(1→ and decorated by sulfate groups and α-l-Fuc branches. Two polysaccharides were different in pattern of sulfation of GalNAc and fucosyl branches connected to O-3 of GlcA. The ratio of GalNAc4S6S:GalNAc4S for AJ was about 2:1, whereas for AM this value was approximately 1:1. AJ contained Fucp2S4S and Fucp3S4S residues linked to O-3 of GlcA in a ratio of 3:1, while for AM this ratio was 1:4. Small portions of Fucp4S units attached to O-3 of GlcA were also found in both polysaccharides. Moreover, in a structure of AM the presence of Fucp3S residues linked to O-6 of GalNAc were determined using the data of NMR spectra.