Search for the origin of paralytic shellfish toxins occurring in coral reef crabs and gastropods resulted in a discovery of a calcareous red alga Jania sp. as the toxin progenitor. The alga was confirmed to contain gonyautoxins I, II and III, the first being the major component. The presence of this alga in the stomach of two toxic crabs Zosimus aeneus and Atergatis floridus and four gastropods Turbo marmorata, Turbo argyrostoma, Tectus pyramis and Tectus nilotica maxima evidenced that Jania sp. was the primary source of the toxins. Seasonal observation indicated that the toxin content in the alga reached the maximum in the spring.