Morphological, cytological, and isozyme studies of three plants of Polystichum fibrillosopaleaceum var. marginale (Dryopteridaceae) from Gotenba, at the foot of Mt. Fuji, Shizuoka Prefecture, Japan, and comparative observations of their lamina, scales, sori, spores, and perispore ornamentation, revealed them to be intermediate between P. fibrillosopaleaceum and P. igaense. Cytologically, the meiotic chromosome number of n = c. 38II 6I (2x) and the malformed spores of the three plants are characteristic of hybrid sterility. The electrophoretic polymorphisms of four isozymes (Pgi, Pgm-1, Pgm-2, and Skdh) were examined in the three plants and in 22 individuals of the putative parental species, P. fibrillosopaleaceum (11) and P. igaense (11). The heterozygous genetic patterns (Pgi-ab, Pgm-1ab, Pgm-2ab, and Skdh-ab) fixed in the putative hybrids indicate that they have a combination of the Pgi-b, Pgm-1b, Pgm-2a, and Skdh-a alleles of P. igaense and Pgi-a, Pgm-1a, Pgm-2b, and Skdh-b alleles of P. fibrillosopaleaceum. The morphological, electrophoretic features and chromosome numbers confirm the three plants to be diploid hybrids of P. fibrillosopaleaceum and P. igaense. We therefore conclude that P. fibrillosopaleaceum var. marginale is in fact a diploid hybrid and thus should not be given the variety status.