The genus Ligia (Ligiidae) has a worldwide distribution and currently includes more than 30 nominal species. Most of the species are littoral, halophilic, and occur on rocky seashores, but seven species are strictly terrestrial. Three species of Ligia have been recorded from the Hawaiian Islands, one introduced, L. exotica, and two edemic, L. hawaiensis and L. perkinsi. Ligia hawaien-sis is a littoral species very common along the rocky coasts of the Hawaiian Islands, and Ligia perkinsi is a montane terrestrial species occurring on Kauai, Oahu, and Hawaii. Morphological and molecular data of the endemic species of Ligia from the Hawaiian Islands are used to test whether the adaptation to the terrestrial environment took place only once or, conversely, it evolved on each island independently. Populations of L. hawaiensis and L. perkinsi from Kauai and Oahu were examined. Four non-Hawaiian species of Ligia (L. italica, L. pallasii, L. vitiensis, and L. ex-otica) and the related ligiid Ligidium hypnorum were included in the molecular analysis as an out-group. All populations of L. hawaiensis were found to be morphologically identical to each other and distinctly different from L. perkinsi. The Kauai populations of L. perkinsi differ slightly from the Oahu one in morphology and ecological habitat. Two regions of mitochondrial DNA were se-quenced: 675 bp of cytochrome c oxidase subunit I and approximately 490 bp of 16S rRNA. The cladogram obtained shows that L. hawaiensis and L. perkinsi belong to the same clade and that L. perkinsi does not constitute a monophyletic unit. The populations of L. hawaiensis show a re-markably high level of geographic structure suggesting that migratory events between the islands are uncommon. Thus, the independent colonization of terrestrial habitat by an ancestral seashore population of Ligia is proposed as the most plausible scenario for the origin of the terrestrial popu-lations.