A comprehensive assessment of bacterial diversity and community composition in Arctic and Antarctic pack ice was conducted through cultivation and cultivation-independent molecular techniques. We sequenced 16S rRNA genes from 115 and 87 pure cultures of bacteria isolated from Arctic and Antarctic pack ice, respectively. Most of the 33 Arctic phylotypes were >97% identical to previously described Antarctic species (Bowman, J. P., S. A. McCammon, M. V. Brown, D. S. Nichols, and T. A. McMeekin. 1997. Appl. Environ. Microbiol. 63:3068-3078) or our own Antarctic isolates. At both poles, the a- and g-proteobacteria, and the Cytophaga-Flavobacterium group were the dominant taxonomic bacterial groups identified by cultivation as well as molecular methods. Diversity was higher in Arctic pack ice and detection of Gram-positive bacteria and b-proteobacteria indicated a terrestrial influence on Arctic pack ice as compared to the Antarctic. The analysis of 16S rDNA clone libraries from multiple Arctic and Antarctic pack ice samples revealed a high incidence of closely overlapping 16S rDNA clone and isolate sequences. Simultaneous analysis of environmental samples with fluorescence in situ hybridization (FISH) showed that ~95% of DAPI-stained cells hybridized with the general bacterial probe EUB338. More than 90% of those were further assignable with approximately 50 and 36% as g-proteobacteria in Arctic and Antarctic samples, respectively, and approximately 25% as a-proteobacteria and 25% as the Cytophaga/Flavobacterium group. For the quantification of specific members of the sea ice community, new oligonucleotide probes have been developed which target the genera Octadecabacter, Glaciecola, Psychrobacter, Marinobacter, Shewanella, and Polaribacter. High detection rates of these groups corroborated the overlap of clone and isolate sequences. Viable counts were high (12.75 ± 4.74 % in Arctic samples) and the molecular data suggest that most of the important sea ice bacteria have been cultured. Despite the presence or absence of terrestrial influence and differences in the seasonality of ice floes (multi-year vs. first-year), the overall consistency of phylotypes and FISH data between Arctic and Antarctic samples reflect the similarity of the sea ice microhabitat at either pole.