The relationship of the antheridogen from Ceratopteris thalictroides (L.) Brongn. (Antheridogen C) to three other known antheridogens, gibberellic acid, and two natural antheridogens, one from Pteridium aquilinum (Antheridogen A) and one from Anemia phyllitidis (Antheridogen B), is investigated. It