The copepod parasite Mytilicola intestinalis was first described by Steuer (1902) from the gut of Mytilus galloprovincialis (Lam.) in the Gulf of Trieste. Monod & Dollfus in 1932 recorded the same species from M. edulis from Marseilles. In 1939 the parasite was first recorded on the German North Sea coast by Caspers near Cuxhaven, and in 1947 by Ellenby from Blyth, Northumberland. It is now widespread along the English south coast in M. edulis, but the distribution still shows some irregularities that are discussed in this paper.

I am grateful to Prof. J. E. G. Raymont for facilities at University College, Southampton, and for the use of a research table at Plymouth; to Mr F. S. Russell, F.R.S., and the staff of the Plymouth Laboratory of the Marine Biological Association for their assistance; and to Dr D. P. Wilson for facilities to collect at Exmouth. Dr H. A. Cole and Mr J. N. R. Grainger have kindly given me information from their papers not yet published. Dr D. J. Crisp and Dr H. G. Stubbings have assisted by sending me several samples of mussels, and I am grateful also for information received from several other friends named in the text.