Swedish populations of the small brackish-water periwinkle Littorina tenebrosa were examined over a micro-environmental cline. Canonical variate analysis of shell parameters showed a gradual change from typical L. tenebrosa phenotypes to those resembling specimens of L. saxatilis from sheltered boulder shores. At the gene-allozyme level, the Swedish populations of L. tenebrosa were almost identical with nearby L. saxatilis populations, giving a genetic identity of about 0.99. A comparison of Scottish populations of L. tenebrosa with nearby L. saxatilis gave similarly high identities. It is concluded that tenebrosa is a distinct form of L. saxatilis rather than a species in its own right. The ecotype concept is discussed with reference to the various forms of L. saxatilis and it is recommended that this term be applied to the various ecologically distinct morphs of L. saxatilis in preference to such terms as subspecies or races.