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Stable-isotope probing (SIP) is a culture-independent technique that enables the isolation of DNA from micro-organisms that are actively involved in a specific metabolic process. In this study, SIP was used to characterize the active methylotroph populations in forest soil (pH 3·5) microcosms that were exposed to ¹³CH₃OH or ¹³CH₄. Distinct ¹³C-labelled DNA (¹³C-DNA) fractions were resolved from total community DNA by CsCl density-gradient centrifugation. Analysis of 16S rDNA sequences amplified from the ¹³C-DNA revealed that bacteria related to the genera Methylocella, Methylocapsa, Methylocystis and Rhodoblastus had assimilated the ¹³C-labelled substrates, which suggested that moderately acidophilic methylotroph populations were active in the microcosms. Enrichments targeted towards the active proteobacterial CH₃OH utilizers were successful, although none of these bacteria were isolated into …