In order to estimate the degree of implantation of excavating sponges in the oyster populations from the littoral zone at Blanes (north-western Mediterranean Sea), a representative sample of oysters (Ostrea edulis) was collected from depths between 0.5 and 4 m in 1986. All oysters were infested by the excavating sponges Cliona viridis and Cliona celata. Infestation affected always and exclusively the lower valve always reaching the inner side. The degree of infestation proved to be independent of oyster size and weight. Colonization by contact was considered the main means of dispersion of these sponges within the oyster population. Cliona viridis and C. celata emerged as the single excavating species able to effectively compete with other non-excavating or excavating organisms when trying to colonize the particular calcareous substrate provided by oyster shells in the north-western Mediterranean Sea.