Marine animals1-6have long been known to produce acute dermatitis on contact with the human skin. The eruption described herewith, however, is the first (to our knowledge) to be due to contact with a marine plant, a seaweed. This dermatitis has been proven experimentally by one of us (Col. Grauer) by patch testing to be produced by contact with a marine blue-green alga7,8which has tentatively been identified as Lyngbya majuscula Gomont9(Fig. 1). During July and August, 1958, an acute dermatitis, previously not reported in Hawaii, occurred in swimmers frequenting windward beaches on the island of Oahu. During this period more than 125 cases received treatment for this disorder and hundreds of mild, unreported cases were suspected. Cases occurred at beaches from Laie and Kaaawa to Lanikai and possibly Waimanalo, with no instances of occurrence in Kaneohe Bay (Fig. 2). In the experience of one of