1. This thesis describes the investigations and tests carried these have been directed towards solving the problem of the identity of the adult stage of C. patellae, a digenetic trematode parasite of the limpet, Patella vulgate. 2. Morphological descriptions of the trematode parasites (Cercaria patellae and Cercaria B) of the limpet are given and brief comments of those found in the American mud-snail, Naasa obsoleta, have been included. An ecological arid statistical survey on the occurrence and distribution of the parasites of the limpet has indicated that the final host of Cercaria patellae is a bird; in addition, evidence has been presented to show that the metacercariae found probably develop from Cercaria B. 3. A brief survey is reported of flukes found in the birds examined - those likely to harbour the adult form of Cercaria patellae. 4. The serological methods used in this investigation are reviewed historically, and, the chief technique for measuring the precipitin, reaction - photo - electric estimation and the ring test - are described. 5. Antigens used for the purposes of injections and teats have been prepared variously and the role of lipoids in the specificity of the reaction is discussed briefly. Some of the methods, used toy other workers, to estimate the strength of an antigen have been compared, and the moat suitable used here for standardisation of the ring test. The methods used for preparation, of antisera to a variety of cercariae are described. Rabbits and domestic fowl were used, and it appears that domestic fowl are the more efficient antibody-producers in this particular field of serology. It has also been shown, although, not conclusively that better antisera are produced during an active infection, than by a series of injections of dead material. 6. The tests carried out are divided into qualitative and quantitative techniques. The former include observations on living carcariae in normal sera and antisera, employment of the anaphylactic reaction and, finally, use of the ring test. These tests were employed for the purpose of judging the efficiency of different methods of producing antisera. 7. Quantitative tests were applied to two adult trematodes against two selected antisera to cercariae. In an attempt to apply the ring test technique to the ontogeny of C.patellae.