The high concentration (relative to seawater) of arsenic in many marine animals eaten as human food has stimulated interest in the cycling of arsenic in the marine environment. Although arsenic is present in arsenobetaine ((CH/sub 3/)/sub 3/As/sup +/CH/sub 2/COO/sup -/) in the wester rock lobster (Panulirus cygnus), the dusky shark (Carcharhinus obscurus) and the school whiting (Sillago bassensis) it is not clear what intermediate stages are involved in the biosynthesis of this compound from arsenate, the major form of arsnenic in seawater. We now report the isolation of the two main arsenical constituents of the brown kelp, Ecklonia radiata, and their identification as a 2-hydroxy-3-sulphopropyl-5-deoxy-5-(dimethylarsenoso)furanoside and a 2,3-dihydroxypropyl-5-deoxy-5-(dimethylarsenoso)furanoside. A ..beta..-ribo structure for the sugar system is strongly indicated in each case. Ecklonia is the major organisms that concentrates arsenic in the coastal ecosystem to which the western rock lobster and school whiting belong. It is clear that the compounds described here could readily be further metabolized to arsenobetaine and may well be the source of arsenobetaine in marine fauna associated with the region.