The life cycle of Schistocephalus solidus had been known for over thirty years but there was little information in the literature on the maintenance of the life cycle in the laboratory. It was thus decided to obtain each stage of the life cycle under laboratory conditions and study their development. The data obtained is recorded in Section I. The plerocercoid stage is the only stage in the cycle which can be obtained easily in nature. Hence numerous visits were made to two lochs to collect infected 3-spined sticklebacks. Records were kept of all the fish caught and the infection they harboured in order that information relating to the cycle under natural conditions could be obtained. This information is recorded in Section II. The regular sampling of two lochs produced many small plerocercoids which it was found could not become established in a warm-blooded host. The cultivation in vitro of these small plerocercoids was thus commenced as well as the cultivation of any larger plerocercoids not required for feeding to potential definitive hosts. The results of this work are recorded in Section III.