Saxitoxin (1) and several other potent neurotoxins (2,4,6-8) have been isolated from shellfish and dinoflagellates of the genus Protogonyaulax. We have recently described a novel class of related substances, significant in that they have relatively low in vivo toxicity until they are hydrolyzed to the previously known toxins, and have now established the structures of two of these as carbamoyl-N-sulfo-11a-hydroxysaxitoxin sulfate (3) and the 11β-epimer (5). The structure determinations of these new substances are of particular interest since (a) compounds 3 and 5 are the most widespread and abundant toxins produced by dinoflagellates found along the Alaskan coast from Southeast Alaska to the Aleutians, (b) this is the first report of the N-sulfocarbamoyl group in a natural product, (c) the attenuation of toxicity associated with sulfonation of the carbamoyl group introduces a new aspect of structure-activity relationships in this group of neurotoxins, and (d) establishment of structure 5 by X-ray crystallography coupled with the chemical interconversions described herein confirms the structural assignments for 2 and 4 previously made principally from spectroscopic data.