Intertidal seawalls in Sydney Harbour are home to various molluscs, but their abundance and diversity vary. Sessile bivalves and limpets are found in similar numbers on seawalls and natural shores, while coiled snails and opisthobranchs are more common on natural shores. Common mollusc species are found in similar numbers in natural and artificial boulder-fields, but rarer species are not present in these boulder-fields. These findings have important implications for managing intertidal biodiversity in urbanized areas.