Karyological studies were made on Japanese ferns of 2 genera, 4 species belonging to the Hymenophyllaceae. Chromosome numbers of the 4 species are as follows; Hymenopyllum barbatum 2n=42, Mecodium oligosorum 2n=42, M. wrightii 2n=84 and M. polyanthos 2n=56. The chromosome complements of the species in Hymenophyllaceae studied by the authors can be divided from the morphological view point into 6(A-F) types and further can be divided into 2 groups in each type except for F type. Therefore, the primary basic number of the genus Hymenophyllum and Mecodium can be considered to be x=11. After polyploidizing, one part of B type elements in genus Hymenophyllum and one part of C type elements in genus Mecodium might be subjected to elimination which leads to the present chromosome numbers.