Significant differences in the infection of M. edulis and the “Padstow type” mussel with P. pisum are recorded, and some possible explanations for these differences are discussed.

Both types of Mytilus from the mid and lower regions of the mussel bed showed heavier infections than mussels higher on the shore. Even so, the differences between the two types were still maintained.

A relationship exists between crab and mussel size, larger crabs being found only in larger hosts. The smallest mussel found to be infected with Pinnotheres measured 3·35 cm in length.

Infection in M. edulis was found to increase with increased size of host, the largest occurring mussels having from 80 to 100% infection. Larger mussels occurred in greater numbers in the low shore. It is assumed that infection in the “Padstow type” would show a similar relationship if sufficient recordings had been available.

The presence of the crab causes gill damage, and infected mussels show considerably lower tissue weights and slightly greater shell weights than uninfected mussels of similar size.

The presence of the crab does not appear to influence the reproductive capacity of the mussel.