Spores of the fern Ceratopteris richardii can develop as either males or hermaphrodites; the difference in sex is determined by the pheromone antheridiogen secreted by the hermaphrodite. During spore germination, the first spores to germinate are hermaphrodites, which will secrete antheridiogen into the surroundings. Antheridiogen will induce later germinating spores to develop as males. Previous studies have shown that antheridiogen is necessary for both induction and maintenance of males. In the present study, fully developed males were isolated from hermaphrodites in order to test the effects of various parameters on their conversion to hermaphrodites. The results show that in the absence of antheridiogen, males converted to hermaphrodites. However, males exposed to higher levels of antheridiogen converted more slowly than males exposed to lower levels. Furthermore, males exposed to antheridiogen for an extended period converted at a lower frequency than males exposed for a shorter period. When test spores were sown in the presence of converted males, all germinated and developed as males, indicating that converted males have the ability to secrete antheridiogen as do normal hermaphrodites. Finally, converted males were able to self-fertilize and produce healthy, fertile sporophyte offspring.