Unlike the results of other comparative approaches, random amplified polymorphic DNA patterns from sympatric populations of Littorina saxatilis and L. arcana show clear differences. Analysis of the data using the method of Nei & Li (1979; Proc. natn. Acad. Sci. U.S.A. 76:5269-5273) separates individuals of the species into 2 discrete groups, supporting the view that their species status is justified.