Abstract By x-ray crystallography of the 11β epimer, toxins C3 and C4 are shown to be 21- sulfo-N-1-hydroxysaxitoxin-11α-and 11β-hydroxysulfate, confirming the position and identity of the 3 substituents which, with the parent compound, form the array of 12 saxitoxins found in Full-size image (< 1 K)