(2s) -3- [5-Deoxy-5- (dimethylarsinoyl) -p-o-ribofuranosyloxy] -2-hydroxypropyl hydrogen sulphate, (1 a) and 3- [5-deoxy-5-(dimethylars~noyl)-~-~-r~bofuranosyloxy]propy~egnlyec ol, (1 b), have been isolated from the kidney of the giant clam, Tridacna maxima, collected from Shark Bay, Western Australia. The structure of compound (1 a) was determined by X-ray diffraction. The source of the arsenicals (1 a) and (1 b) is likely to be symbiotic, unicellular, green algae living in the clam tissues. as arsenic-containing sugars [including compound (1 b)] have previously been isolated from Ecklonia radiata (a macroalga), it is possible that the production of such compounds is a general response of algae to oceanic arsenate.