This thesis investigates the systematics and population genetic structure of all known wave-exposed northeastern Pacific Littorina using DNA sequence data and single stranded conformational polymorphisms (SSCP) from the mitochondrial DNA cytochrome-b gene. The data confirmed that there were four Littorina species, two direct-developing, species: L. sitkana and L. subrotundata and two with planktonic larvae: L. scutulata, and L. plena. AMOVA revealed that the wave-exposed form and the salt-marsh form of L. subrotundata were ecotypes and not separate species which have most likely arisen by parallel evolution in each geographic area. The population genetic results for L. scutulata and L. subrotundata followed the general paradigm in that the former species has planktonic larvae and shows less population genetic structure than the direct developing species. In contrast, L. plena revealed some significant population structure over a limited geographic area.