Microbial diversity, numbers, and metabolic activities in Minnesota peatlands were investigated using a variety of microbial enrichment and enumeration procedures together with radioisotopic measurements of microbial degradative processes. Minnesota peatlands were shown to contain large microbial populations of wide metabolic diversity. Direct counts of bacteria using epifluorescence microscopy indicated bacterial populations of about 10(8) ml(-1) of peatland water, irrespective of depth. Radioisotopic most-probable-number (MPN) counts of heterotrophs able to mineralize(14)C-labeled substrates to(14)CO2 showed significant populations of glucose degraders (10(4)-10(6) ml(-1)) as well as degraders of benzoate (10(2)-10(3) ml(-1)), 2,4-dichlorophenoxyacetate (10(2)-10(5) ml(-1)), and sphagnum (10(3)-10(7) ml(-1)) in the various peatlands examined. The MPNs of NO3 (-) reducers varied from 10(3)-10(6) ml(-1), SO4 (-) reducers from 10(2)-10(3) ml(-1), methanogenic bacteria from 10(3)-10(6) ml(-1), and methane oxidizers from 10(3)-10(4) ml(-1), depending on sampling site and depth. Eighty pure cultures of aerobic bacteria and fungi were isolated from Minnesota peats. Most of those cultures tested were able to grow on at least 20 organic compounds (carbohydrates, aromatic molecules, hydrocarbons, etc.) as sole sources of carbon and energy. One isolate, aBacillus, was able to fix atmospheric N2. Several of the isolates were able to mineralize(14)C-labeled lignin.