Biological and physico-chemical characteristics of subglacial sediments were studied in Svalbard. Sediment from close proglacial and supraglacial environments was used for a comparison. Viable bacteria, cyanobacteria and microalgae were detected in subglacial sediments from two polythermal glaciers using epifluorescence microscopy and phospholipid fatty acid (PLFA) analyses. The subglacial samples were generally of higher pH values, coarser texture and lower water content, organic matter, organic carbon, and nitrogen compared to proglacial and supraglacial sediments). Bacterial counts of 1.6 x 10(7) cells mg(-1) OM (organic matter) were found. Cyanobacteria and algae were also of low abundance [4.2 cells mg(-1) DW (dry weight)]. Cyanobacteria comprised the major proportion of the autophotothrophic assemblages of subglacial soils. Deglaciated soils were similar to subglacial sediment in physico-chemical properties and microbial structure and numbers, unlike soil from vegetated sites or cryoconite sediment. In subglacial and deglaciated soil, relatively low diversity of microorganisms and low substrate availability was detected by PLFA analyses. Good accordance in microbial community structure assessments between epifluorescence microscopy and PLFA analyses was found. Our results suggest that the subglacial microbial populations can be divided into two groups: autochthonous microorganims (chemoheterotrophic bacteria) and allochthonous that retain the ability to proliferate and give rise to active population when conditions become favorable.