ABSTRACT: Research on the impact of mass aggregations of jellyfish on foodweb structure has mainly focused on trophic control of copepods and fish larvae, while impacts on lower trophic levels have received little attention. Jellyfish release nutrients and dissolved organic matter through their activities. Hence, both direct and cascading impacts of jellyfish on the bacterial community are conceivable. In the Norwegian Lurefjorden, the abundance of the deepwater scyphomedusa Periphylla periphylla reaches concentrations much higher than in any other area investigated. We used Lurefjorden as a model system to examine effects of high jellyfish biomass on the microbial planktonic foodweb, and targeted bacterial activity and community composition in relation to the distribution of P. periphylla as monitored using a Remotely Operated Vehicle. P. periphylla performed pronounced diel vertical migrations; however, no significant diel effects on microbial activity were observed in surface waters. Surface waters were characterized by a high biomass of mesozooplankton. Integrating over 24 h, the highest jellyfish biomass was found at 200 to 300 m depth. This concurred with elevated total organic carbon, bacterial production and ectoenzymatic activities. Analysis of bacterioplankton community composition by denaturing gradient gel electrophoresis (DGGE) and 16S rDNA sequencing revealed that specific phylotypes related to Bacteroidetes and to δ-Proteobacteria were found with depth. Overall, phylotypes related to Bacteroidetes dominated the bacterial community. Our results indicate that P. periphylla has a structuring impact on the pelagic deep-water microbial community in Lurefjorden. We suggest that jellyfish proliferations may also be quantitatively important for lower trophic levels of the pelagic foodweb.