Portland Harbour is under increasing recreational and developmental pressure and this paper was intended to place on record at least one aspect of its high marine biological importance. The shores and sublittoral are described, the present knowledge of its molluscan fauna is stated, and its 140 years of mollusc recording is summarised in relation to the history of enclosure of Portland Roads. The sandflats at Smallmouth and Sandsfoot are the only undisturbed sediment shores between Studland and Exmouth; they are particularly important for their unusually rich and diverse bivalve community, for several species normally sublittoral, and for a colony of a rare and local sea slug (Aolidiella alderi). The double low tide particular to Weymouth Bay is also mentioned.

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