The microbial diversity of the unique high-temperature vent field east of Grímsey island, North of Iceland, was examined for the first time by in situ colonization experiments and molecular analysis of the obtained biomass. Two in situ colonization supports with and without nutrients were deployed parallel  at 394 m within the upper layers of a sediment-rich vent field over 53 h, in a zone of percolating 50°C to 80°C hydrothermal fluid. Sequence analysis of 16S rRNA clones indicated that a majority of putatively hyperthermophilic, thermophilic and moderately thermophilic Bacteria (53 phylotypes) and Archaea (14 phylotypes) were recovered, scattering through several taxonomical divisions. The most abundant clones in the Bacteria libraries belonged to the class ε-Proteobacteria (36 phylotypes, 84% of 219 clones) in both colonization experiments.  Other phylotypes corresponded to other subclasses of the Proteobacteria, Bacteroidetes, Actinobacteria, Deinococcus-Thermus, high and low G+C Gram-positives, Desulfurobacterium and Aquificae but were mostly (16 out of 17) detected within the biomass obtained from the nutrient added support.  Furthermore, only growth of Archaea was detected within the biomass from the nutrient added support. Archaea was dominated by members of the Thermococcales (9 phylotypes, 91% of 102 clones) followed by few phylotypes belonging to the Archaeoglobales, Thermoproteales, uncultured Crenarchaeota and Korarchaeota. This study describes a simple but effective biomass collector for deep sea research and is the first report on the microbial diversity at the Grímsey hydrothermal vent field.