A comprehensive assessment of diversity and community structure in melt pools on Arctic pack ice floes revealed a predominance of b-proteobacteria. Thirty-five percent of the pure cultures isolated in 1997 from pack ice floes north of Svalbard and in the Fram Strait were identified as b-proteobacteria, however diversity within this group was limited to only 2 phylotypes that clustered within the widespread Comamonadaceae clade. One phylotype, most closely related to soil ultramicrobacterium ND5 (96.4-97.0% identical), was frequent among cultures isolated from 10 melt pools. A 16S rRNA gene clone library, constructed from a melt pool sampled 2 years later in the Fram Strait, also revealed predominance of b-proteobacteria, in particular the same recurrent isolate phylotype. Fluorescence-in situ-hybridization of 20 melt pools corroborated the cultivation and cloning data. b-proteobacteria were the most abundant bacterial group constituting approximately 49% of DAPI counts. a- and g-proteobacteria accounted for only 2% each, the Cytophaga-Flavobacterium group for 9% and the Actinobacteria for 9%. Approximately 63% of the b-proteobacterial fraction in the melt pools was determined with a newly developed probe to be the recurrent b-proteobacterial phylotype. The molecular signature of the melt pools was reflective of freshwater or terrestrial ecosystems and appeared to represent a distinct population separate from the bacterial community found within the sea ice matrix.