Gametophyte morphologies and gametangial sequences in agar cultures of Bommeria species indicated developmental control by an antheridiogen system. The first gametophytes to develop were meristematic and archegonial with ameristic, exclusively antheridial plants appearing later in the same cultures. Bommeria gametophytes were found to form precocious antheridia in response to antheridiogen A but did not respond to antheridiogens B or C. Assays of the native Bommeria antheridiogen indicate that it is an A type antheridiogen hormone and that each Bommeria species responds to the hormone produced by its congeners. Differential response by Bommeria species to dilute antheridiogen may be useful in concert with morphological characters in establishing relationships among species. Spores which establish colonies of unisexual gametophytes and fail to produce sporophytes in monospore culture suggest the presence of an obligately outcrossing breeding system. Such a breeding system, while maintaining genetic diversity, would restrict range extension of the sexually reproducing species. Only the apogamous Bommeria pedata is capable of establishing a new sporophyte plant by a single spore.