Toxic Red Tides are caused by marine dinoflagellates which synthesize neurotoxins that accumulate in bivalve mollusks. Upon ingestion, these shellfish can cause paralysis and death. During the month of December, 1989, 99 cases of Paralytic Shellfish Poisoning in the areas of Salina Cruz and Huatulco, Oaxaca, were reported; three of which died. The majority of the cases involved the ingestion of oysters and clams. A short time later, the Red Tide was reported appearing. One hundred and fifteen seafood samples were analyzed through a biologic test in mice to determine the quantity of saxitoxin. Oysters and clams showed levels reaching 4 000 U.R. Laboratory analysis of seawater found very high quantities of Gimnodinium catenatum and Gonyaulax catenella. The control measures consisted of the prohibition of the harvest and sale of all bivalve mollusks as well as a public warning to avoid the consumption of such shellfish.