Homosporous ferns produce only one type of haploide spores, however each of them is sexually bipotential and can develop as a male or hermaphrodite gametophytes. Their sex is often deter- mined by an epigenetic mechanism that is responsive to antheridiogens which are secreted by hermaphro- dites and promote the development of males from sexually undetermined gametophytes. Analyses of sex determination in the gametophytes of Ceratopteris richardii, one of the model fern species, has been defined a hypothetical network of at least eight interacting genes involved in sex expression with connec- tion of endogenous antheridiogen. However, in the gametophytes of Anemia phyllitidis, the second impor- tant model system in the studies of sex determination, male sex could be induced by gibberellic acid which imitates the action of antheridic acid, the main antheridiogen of that fern. Gibberellic acid produces specific cytomorphological effects which allowed to create the three zone model of A. phyllitidis gameto- phyte structure. The results of recent analyses have revealed that ethylene involved in controlling sex determination in A. phyllitidis. It seems to mediate the action of gibberellin in process of sex expression. In this paper the hypothetical mechanism of this activity is proposed.