Stream net samples in the Esk estuary reveal the infection, in winter, of the Gammarus zaddachi zaddachi Spooner, 1947 (Crustacea, Amphipoda) population alone with the plerocercoids of Diplocotyle sp. (Eucestoda, Spathebethriidea). Large plerocercoids are found in both sexes of the gammarids but the parasitized females are significantly larger than the non-infected ones and their fertility may be reduced.
I wish to express my thanks to Professor I. Chester-Jones of the University of Sheffield Department of Zoology in whose laboratories this work was carried out, and to Dr F. Segrove (of the same Department) for advice and constructive criticism.