Female and male prothallia were developed from the spores of Ceratopteris pteridoides grown in culture. The female prothallia remained female throughout the investigation, but the male prothallia underwent further development to produce cither female parts to become hermaphroditic or adventitious female prothallia. Most of the prothallia were insensitive to external GA», but some male prothallia were promoted by this growth hormone to produce prothallial extensions which bore archcgonia. GAj might possibly be an endogenous hormone opposed to anthcridiogcn, promoting the growth and feminization of the prothallium. ABA inhibited antheridial formation and prothallial growth as well.