The seven currently recognised New Zealand species in the gastropod genus DilomaPhilippi, 1845 are an important component of New Zealand's littoral biodiversity across a range of intertidal habitats. A new cryptic species in this genus (Diloma durvillaea, sp. nov.) is described largely on molecular grounds from exposed coasts of the South Island of New Zealand from Lyttelton south, as well as the Auckland Islands. The shell is very similar to D. arida(Finlay, 1926), differing subtly in having stronger spiral ribs and less extensive or no yellow spotting. Phylogenetic analysis of the mitochondrial cytochrome coxidase I (COI) gene shows these two species are sister-taxa and are more distinct from each other (genetic distance 11.5%) than are the morphologically divergent sister-species D. subrostrata(Gray, 1835) and D. aethiops(Gmelin, 1791) (8.2%), also from New Zealand. The new species is found in a novel habitat for the genus, in the holdfasts and on the blades of the bull kelp, Durvillaea antarctica(Cham.) Har. (Phaeophyta), at the low-tide mark on exposed coasts, whereas D. aridaoccurs higher up on the shore, as well as in more sheltered situations. [ABSTRACT FROM AUTHOR]