ABSTRACT: During a field mesocosm experiment conducted in coastal waters off western Norway, 11 m³ enclosures were filled with unfiltered seawater and enriched daily with different nitrate and phosphate concentrations in order to induce a bloom of the coccolithophorid Emiliana huxleyi under different nutrient regimes. Flow cytometry (FCM) analysis was performed 5 times d^-1 in order to follow the initiation, development and termination of the bloom as well as the production of large virus-like particles (LVLPs) identified as E. huxleyi viruses (EhV). EhV production was observed first in the enclosure where N was in excess, and P limitation induced a lower burst size compared to nitrate-replete enclosures. These observations suggest a critical role for both P and N in E. huxleyi-EhV interactions. Concomitant to EhV production, a shift was observed between the original population (coccolith-bearing cells) towards a population characterized by the same chlorophyll a (chl a) fluorescence but with lower right angle light scatter values. This population is likely to correspond to either senescent cells losing their coccoliths or cells characterized by a lower production of coccoliths possibly due to viral infection. At the end of experiment, a significant proportion of E. huxleyi had survived after the end of the bloom. This suggests either the presence of a resistant form of the coccolithophorid or a change in the dominance of different host and/or viral strains during the bloom. A periodical pattern in virus production was recorded with virus number decreasing during the second part of the day suggesting that virus production may be synchronized to the daily light cycle. Our results provide new insights towards the understanding of the relationship between a key marine species and its specific virus.