<jats:p> A highly inflammatory and vesicatory substance, lyngbyatoxin A, has been isolated from the lipid extract of a Hawaiian shallow-water variety of <jats:italic>Lyngbya majuscula</jats:italic> Gomont; its gross structure was determined from chemical and spectral data. Lyngbyatoxin A is closely related to teleocidin B, a poisonous substance associated with several strains of <jats:italic>Streptomyces</jats:italic> .