Several dinoflagellate species in the genus Blastodinium are gut parasites of marine planktonic copepods. However, there is only limited information on the occurrence and infection frequencies of Blastodinium spp. in the field and almost no information on the functional impact on their hosts. We report upon the effects of Blastodinium sp. infection on Calanus finmarchicus from the northeastern Atlantic coast off southern Norway during April 2013 and 2014. Up to 58% of C. finmarchicus were infected near the coast, while <5% were infected several kilometers offshore. Ingestion rates of infected females were below detection limits and significantly lower than uninfected females. Blastodinium sp.-infected females showed characteristic symptoms of starvation, including lower respiration rates (implying a lower metabolic rate), production of smaller and fewer fecal pellets and significantly fewer eggs than uninfected females. A few females in this study were able to void the infection, however the extended period of starvation is likely to have longer-term repercussions on egg production rates well after the copepod clears the infection. The degree to which the infection affects C. finmarchicus recruitment depends on the extent of the spatial distribution of the infection. Monitoring of parasitic infection during routine field surveys will be required in order to clarify this.