During sea-ice formation a fraction of the dissolved salts, other solutes, microbial cells and particles present in the source water is expelled from the ice. Recent field data indicate that algal and heterotrophic protists, extracellular polysaccharide substances (EPS) produced mainly by algae and, to a lesser extent, bacteria are selectively retained in the ice relative to the salts. The enrichment of bacterial cells in newly formed sea ice has been considered a secondary effect of their interactions with algal cells or algal EPS. We propose that EPS of bacterial origin may contribute directly to the selective retention of bacteria (those with EPS-coated surfaces). To test this hypothesis we compared the retention in saline ice of EPS produced by the cold-adapted gamma-proteobacterium Colwellia psychrerythraea strain 34H to the retention of salts in the same ice. Additional experiments tested the comparative retention of Colwellia EPS first heated to denature its heat-labile components and of EPS of eukaryotic (yeast) origin initially present in the growth medium (also subjected to heating during its preparation). The saline ice was formed by means of a coldfinger apparatus, where a glass tube kept at –5°C (by circulating antifreeze) was placed into artificial sea-water solutions containing the different types of EPS. Calculations of enrichment indices (EPS relative to salts) for each set of experimental conditions, conducted in triplicate, revealed that only the native (unheated) bacterial EPS was enriched in the ice. Not only is the EPS of strain 34H retained preferentially, but the enrichment of this material in saline ice depends on the presence of a heat-labile fraction most likely of protein origin. Because the whole genome sequence of strain 34H does not reveal the presence of genes for known ice-affine proteins, the biochemical basis for the detected EPS enrichment is not yet clear. Other data obtained from our experiments suggest that the selective retention of EPS of bacterial origin contributes to physical alteration of the ice habitat such that, altogether, our results yield an improved understanding of how EPS may help to establish and sustain the microbial community that inhabits sea ice.