The biology of Secculina carcini, a parasite on the common shore crab, Carcinus maenas, was studied with particular emphasis on larval morphology, biochemical content and energetic demands. The prevalence and distribution of the parasite was also investigated. The field studies carried out on the populations of S. carcini on C. maenas in northern France and North Wales, together with a review of the literature, demonstrated a disparate distribution. The larvae of S. carcini are lecithotrophic and can develop within 5 days to the cyprid and are capable of settlement 2 days later. The highest prevalence for the parasite was observed to correlate with semi-enclosed bodies of water. It is proposed that in this situation, with the short development time, the larvae are retained, thus increasing the probability of successful cypris settlement. Such coastal systems are susceptible to variable conditions, consequently experiments were undertaken to investigate the larval tolerance to temperature and salinity variations. A morphological study was carried out on the naupliar stages of S. carcinl. The typical cirripede limbs were simple and lacked gnathobases. There was also a vestigial ventral thoracic process present in the stage III and stage IV nauplius. These observations supported the argument that the rhizocephalan nauplius fits within the cirripede nauplius form. - ii - The energy budget study, involved the investigation of ash-free dry weights per larva, respirometry for each larval stage and analyses of the biochemical constituents using colorimetric and gravimetric techniques, Finally an equation was developed which demonstrates the energy budget for S. carcini, from the beginning of larval development to settlement on the host and the subsequent metamorphoses to the inoculation stages.