Three sibling species of rough periwinkles are currently recognized: Littorina arcana, L. compressa and L. saxatilis. Certain forms of L. saxatilis are also argued by some to deserve species status, such as the barnacle-dwelling ‘L. neglecta’ and the lagoonal ‘L. tenebrosa’. Relationships between these taxa, and between and within representative populations, are investigated using sequence analysis and restriction fragment length polymorphism of a mitochondrial DNA fragment spanning the cytochrome oxidase I and cytochrome oxidase II gene boundary. These data show that there is some sharing of haplotypes between species, with L. arcana haplotypes paraphyletic with respect to L. saxatilis haplotypes, and L. compressa haplotypes paraphyletic to both L. arcana and L. saxatilis haplotypes. Such sharing of mtDNA haplotypes could be a consequence of either persistent hybridization or episodes of hybridization, or incomplete lineage sorting of ancestral polymorphisms. On the balance of evidence it is suggested that the latter, rather than hybridization events, is the more likely causal agent of the observed distribution. Intraspecific variation is extensive and it is suggested that the patterns of intraspecific polymorphism are explainable by a combination of historical factors (the impact of the Pleistocene ice-age) and contemporary restrictions to gene flow. It is argued that Littorina haplotypes evolved in at least two separate glacial refugia and became scattered by the subsequent range expansion around most of the coastline. Recent factors such as restricted gene flow and bottlenecks would then be capable of affecting the haplotype distribution, resulting in the pattern observed.