Diellia is a monophyletic genus that has radiated out over the six Hawaiian Islands and is currently found on five. As this endemic fern genus consists of very rare and local federally listed endangered species, we aimed to describe as many extant Diellia populations as possible. Diellia leucostegioides has not been found for over 100 years and is listed as the species of concern (US Fish and Wildlife Service Species List 1999). In particular, we included local populations of five taxa in our study: D. erecta, D. pallida, D. falcata, D. unisora, and D. erecta f. alexandri. We omitted D. mannii from further analysis, since it was represented by one individual only, as well as populations with uncertain taxonomy. The local populations of D. falcata, D. unisora, and D. falcata ×unisora (Table 1) in the so-called “hybrid zone” in Pualii–Palawai area on Oahu were not considered. In the case of D. unisora, we checked three sites, but used only one, morphologically almost a “pure” population patch, for further analysis.