A "red tide" bloom of Gonyaulax polyedra occurred in Italy in Autumn, 1988. Algal concentrated extracts and undiluted water samples from the bloom were tested both with the Microtox system and a mouse bioassay, revealing the presence of paralytic shellfish poison-like neurotoxins. Saxitoxin levels evaluated on the basis of toxicological and instrumental analysis showed discrepancies. Other toxins could be present in addition to paralytic shellfish poison.