The present study deals with systematic and taxonomic problems in the acrostichoid fern genus Bolbitis (Lomariopsidaceae). The idea came from Prof. Holttum, Kew, in view of his intended revision of the lomariopsidoid ferns for Flora Malesiana. Originally the study was confined to the Asian representatives. However, various problems remained unsolved after the study of these taxa alone, and it was decided to monograph the genus. The taxonomy of the genus Bolbitis was seriously misunderstood in the past—parallel evolution obscuring true relationships—and a rather intricate synonymy has resulted. A brief historical sketch of its taxonomic adventures is worked out. The gross-morphological criteria used for the discrimination of the species by Copeland (who revised the Asian representatives in 1928) and Ching (who revised ser. Egenolfianae in 1931) could not be applied satisfactorily to the ample material at hand. Therefore, several features of the sporophyte were analyzed in detail, including rhizome- and petiole-anatomy, venation pattern and spore-morphology. The results are described and discussed. Further, the possible occurrence of hybridization and polyploidization was investigated. For this purpose a collection of different species was taken into cultivation and their mitotic and/or meiotic chromosome number and behaviour recorded. A limited number of species could also be studied in the field. From several species significant information on the morphology of the juvenile leaf stages could be obtained. As could be demonstrated the delimitation of the genus as given especially by Ching (in Christensen, 1934) is largely correct, though he included the species with anastomosing veins only. A number of Ching’s species, however, are excluded and referred to non-lomariopsidoid genera. Also, in a few cases, species from other genera are here included in the genus for the first time. As most of the taxa under discussion were formerly included in Leptochilus, this genus was critically investigated as well. The results will be published separately. The genus Bolbitis as here defined and emended includes the genera Egenolfia and Edanyoa. Fourty-four species (one of which new) and 13 hybrids (12 of which new) are recognized, based on morphological criteria. The venation pattern and the spores provide important characters. In some species infraspecific categories are listed. Identification keys to all taxa are included. The genus is subdivided into 10 series which accommodate a total of 34 species; another ten species are separately listed mostly in view of their suspected hybrid origin. Hybrids could be detected because of structural irregularities, especially in spores, but usually also as to other features. Putative hybrids between cytotypes of one species are not separately listed. Generic and/or specific delimitation differs considerably from that given by Ching (in Christensen, 1934), Christensen (1934), Holttum (1954), and Copeland (1960); the latter eventually recognized 97 different species in four different genera ( Stenosemia dimorpha included). Notes are given on specific delimitation. Data are supplied which stress the evolutionary importance of polyploidization, hybridization, vegetative propagation, and the retention of juvenile characters, factors that may explain the often fascinating diversity found within many species. The study of ontogenetic leaf stages (which may be very different from adult ones) has received great attention after it was understood that at least part of the variation as found in the adult sterile leaves recalls features found in the juvenile leaves. The relation between ontogeny and phylogeny is shortly discussed. Some facts are given which support my impression that the retention of juvenile characters may have played an important role during the diversification within the genus. Some general considerations about the probably great general significance of the retention of juvenile characters for the past diversification of the leptosporangiate ferns are included. It is postulated that the recognition ofthis neglected phenomenon may add considerably to our understanding also of the larger groups of leptosporangiate ferns.