On exposed rocky shores in northwestern Spain there is a striking polymorphism of Littorina saxatilis that has been claimed as an example of a putative sympatric ecological speciation process. Two ecotypes of this species have evolved that are adapted to different shore levels and habitats, although they meet and hybridize on the mid shore where their two habitats overlap. As a by-product of adaptation these ecotypes have evolved an incomplete premating reproductive isolation where they meet on the mid shore. Although they are not yet true species, and the final outcome of the process cannot be predicted, the ecological mechanisms responsible for this polymorphism could cause sympatric speciation in similar situations. Here, I review all data in support of these claims and discuss the interest of such a model system in microevolutionary studies.