On 13 November 1947 a specimen of the sea hare, Aplysia depilans L., which had been trawled in Babbacombe Bay, was sent to the Plymouth Laboratory. When it was realized that the animal was not the common A. punctata Cuv., collecting trips to likely places were undertaken in the hope of finding more. No others were found, but on one of the expeditions Dr D. P. Wilson picked up a specimen of A. limacina L.

Both A. depilans and A. limacina are found in the Mediterranean and on the west coast of Europe: A. depilans has been found in British seas before, but so far as is known A. limacina has not.

These occurrences provide the main reason for publishing this study. The paper also includes an account of the distribution of aplysiids in British waters and a review of the controversy over the identity of large specimens. As the animals are not usually described in natural history books, notes on the field characters are added.

I would like to thank the Director of the Plymouth Laboratory for affording me laboratory and collecting facilities and for his interest in the work. I am most grateful to Dr G. Bacci, who went to much trouble to send me specimens from Naples; to Dr W. J. Rees, who arranged for me to have access to the British Museum collection; to Dr D. P. Wilson, who has provided the photographs of A. punctata (Pl. I) and A. limacina (Pl. II); and to D. J. Slinn, who prepared the map.