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This study set out to characterise the fauna and biology of parasites of the barnacle Semibalanus balanoides. The dominant parasite species was identified as a microphallid metacercariae Maritrema sp. and was determined to be the focus of study. Systematic sampling was conducted to investigate the spatial distribution patterns of Maritrema sp. in S. balanoides populations across different latitudes, habitat types and scales; and to explore the factors affecting the discovered patterns. An aggregated or over-dispersed distribution was observed at each sampling site, as well as when the data were pooled. The relationship of variance to mean of metacercarial abundance was found to fit Taylor's Power Law. Genuine spatial heterogeneity was detected after correcting for host size, and a decreased trend of abundance towards the sea was revealed. This trend was more apparent at a gully (local scale) than across whole area scale. Possible causes of this phenomenon were discussed. After analysis of several host and environmental factors, a multiple regression model which explained up to 88% of the variance of abundance was developed. The influence of host density and parasite crowding upon infection parameters were also investigated. Afterward, morphology, SEM, and molecular tools were applied to diagnose this dominant digenean in the studied localities and its identity was confirmed to be Maritrema gratiosum. To confirm that the specimens collected from three different rocky shore localities along the Scottish coast represented a single species, morphometric and molecular methods were applied. Although subtle differences in the morphology of specimens collected from distant localities were found, with single base pair polymorphism being revealed in their ITS2 rDNA, all specimens were considered to be a single species. Distance between sampling localities, culture conditions, and parasite crowding were found to contribute to morphological variations. Finally, an in vitro culture study was performed to obtain ovigerous adults of M. gratiosum. The morphology of the mature forms was observed and the effects of different concentrations of serum upon adult longevity, size and egg production was measured. Survival for 10-days was achieved when flukes were cultured in NCTC-109 plus chicken serum and antibiotics. Forty percent chicken serum seemed to provide better results in terms of survival time and in producing flukes with the largest body lengths. Both normal and abnormal eggs were observed among those produced by the adults cultured in vitro. The ability of self-fertilization was revealed, and the status of progenesis and its evolutionary and ecological significance have been discussed. CLSM was undertaken to provide detailed study of the development of their internal organs and musculature during the course of their in vitro culture. The musculature of M. gratiosum was similar to that of other microphallids, however, some additional novel structures were found, most notably a ligament connecting pars prostatica and seminal vesicle and a racket-shaped excretory bladder. Through the thorough study of a single species of digenean, this study has provided baseline distribution information in the chosen localities, provided greater insight into its biology, and including further insights into its role in rocky shore ecosystems. The methodologies described in the present study can provide a tractable model by which the study of the complexity of coastal host-parasite ecology can be continued.