Adelphocystis aeikineta, a new species of monocystid gregarine, is found in the coelom of the earthworm Keffia variabilis in which the trophozoites occur in large numbers in permanently associated pairs. They are large, measuring 750–1500, μm in length, and exhibit continual peristaltic motion. The ectosarc of the trophozoite is markedly striated and there is no evidence of any anterior differentiation. The gametocytes are rounded and the presumed bipolar sporocysts measure 12–14·5 μm in length. Some associated pairs occur in groups enclosed in a membrane and there is evidence that gametocytes may form under these conditions. A. aeikineta is placed in the subfamily Zygocystinae and separated from Zygocystis on its elongate shape. The site of infection of this gregarine, and others recently described, suggests that the morphological basis on which the family Monocystidae is divided into subfamilies and genera is inadequate and a sounder classification should take into account these sites of infection.

The material on which this paper is based was collected while I was in receipt of a Wellcome Trust Travelling Grant, and I wish to thank the Trust for this support. It is also a pleasure to thank Mr R. W. Sims of the British Museum (Natural History) who identified the earthworms, Dr Keith Vickerman who helped me to collect them and Mr R. B. Freeman who advised me on the nomenclature.