The origin of the side-chain portion of paralytic shellfish toxins has been determined along with some details of the biosynthetic steps. Saxitoxin of an unknown origin was discovered in the common mackerels, Scomber scombrus Linn. A small pennate diatom, Amphora coffaeiformis C1. was identified as a source of a new type of shellfish toxin, domoic acid found in Canadian mussels.