Arctic charr are susceptible to cestodes like Diphyllobothrium dendriticum which are transmitted through the food web. Field studies investigating Arctic charr-Diphyllobothrium relationships often provide little data from which to fully understand Diphyllobothrium transmission or to assess their impact on copepod or fish hosts. Experimental studies may, therefore, be the only direct way to investigate these issues. The research goal of this thesis was to duplicate the D. dendriticum life cycle in the laboratory to investigate parasite development and host specificity, and to apply the experimental data to natural situations. Results indicated that at 10°C, D. dendriticum eggs develop slowly and 65 days are required for complete embryonation. In many Canadian Arctic lakes water temperatures rarely exceed 4°C, and this study has shown that at this temperature embryonation requires several months, necessitating more than one ice-free season for life cycle completion. The results also showed that embryos developed more rapidly and significantly more eggs hatched when incubated with light or aeration, results that were consistent for D. dendriticum originating from Canada and Norway. To continue the life cycle, coracidia must be consumed by suitable copepod hosts. This study showed that although D. dendriticum will infect the European copepods Cyclops scutifer and Eudiaptomus graciloides, prevalence was higher and procercoids developed more rapidly in E. graciloides. These results showed that E. graciloides is a better host for D. dendriticum than is C. scutifer, contradicting published literature. In North America, E. graciloides is not found in lakes containing D. dendriticum and no North American calanoid species have been tested to determine their host suitability to this cestode. This study identified two North American species, Diaptomus minutus and D. leptopus, which are suitable laboratory hosts. D. minutus is almost certainly a natural host for D. den

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