Domoic acid (DA) extracts from 10 specimens of a thorny oyster, Spondylus versicolor collected at Nha Phu bay, Khanh Hoa province, Vietnam in August, 2010 was analyzed individually by High Peiformance Liquid Chromatography (HPLC) and Indirect Enzyme Linked Immunosorbent Assay (ELISA). Domoic acid levels in the extracts detected by ELISA were always higher than those by HPLC, significantly. In the HPLC chromatogram, the peak of an unknown compound with retention time ve,y close to DA, but different from the known DA isomers was always observed. This substance reacted well with DA-specific antibody in Western Blot experiment, but not corresponding to any standard protein. The result indicates the occurrence of a certain non-protein substance, which was co-presented with DA in S. versicolor. Further investigation on chemicalfeature of this substance is under progress.