1. The free-swimming larvae of Mytilicola intestinalis can develop in theabsence of external food. Approximate figures are given for the duration of the free-living and early parasitic stages.

2. Four of the twenty-one samples of Mytilus edulis collected from various parts of Ireland were infected with Mytilicola intestinalis. All the four mussel beds found to be infected were in the neighbourhood of ports.

3. 72·0% of the Mytilicola which were found inhabited the part of the recurrent intestine which is embedded in the digestive gland.

4. There is a significant correlation between mussel length and number of Mytilicola per mussel.

5. There is a significant decrease in the numbers of Mytilicola per mussel between November to December and May to June. It is almost certain that this represents a real decrease in the copepod population, especially in the mussels larger than 55 mm., due to a death-rate which is sufficient to more than offset the increment due to new infection during November to January. In certain chosen size groups of mussels there is also an apparent decrease as a result of mussel growth in the summer, a time when new infection is very small.

6. In the 50–69 mm. mussel group the number of adult Mytilicola per mussel was lowest in September. The number of immature stages was highest in November and December and lowest from May to August. Egg-bearing copepods were found through out the year. There was an excess of males in the adult population throughout the year.

The writer is very indebted to the various people who assisted in collecting mussel samples especially the Irish Sea Fisheries Association; to Miss M. Davidson, B.A., who made a bacteriological examination of a mussel sample; to Dr J. P. Harding for informing him of the specimens of Mytilicola intestinalis in the British Museum collection; and to Prof. J. Brontë Gatenby and Dr J. D. Smyth of Trinity College, Dublin (where this work was carried out), for their suggestions and advice.