During the last decades, the Littorina species became model objects for the study of population biology, physiological adaptations and speciation. These snails are characterized by a high population density, relatively low mobility, and different modes of reproduction. Groups of closely related species form sympatric populations in the intertidal zone, as species of Neritrema subgenus do. Nevertheless, the mechanisms of reproductive isolation are still scarcely investigated. Gamete incompatibility is one of key mechanisms of prezygotic isolation, which is based on the fast evolution of reproductive proteins. Here we report identification and characterization a novel protein from Littorina obtusata sperm - LOSP. This protein has no homologues in databases and no identifiable conservative domains. We detected only minor intraspecies variability of LOSP at transcript level and the possibility of its posttranslational modification. Our results imply a considerable level of interspecies variation, which could refl