A method for the specific detection of microorganisms that assimilate radioactive substrates was combined with the use of membrane arrays consisting of random genomic DNA fragment probes prepared from the original microbial community.The feasibility of this combined method was investigated in terms of specificity, sensitivity and the applicability to the analysis of environmental samples. DNA fragment probes prepared from pure cultures exhibited specific signals to their cognate targets although one probe derived from the rRNA gene was found to be cross-hybridized. Strong correlations of signal intensity with both the amount of probe spotted and the concentration of ¹⁴C-labeled DNA in the hybridization buffer were observed after hybridization with a membrane array using whole genomic DNA as probes. A membrane array was fabricated from an activated sludge sample and hybridized with ¹⁴C-DNA extracted from the same sample grown on either ¹⁴C-acetate or ¹⁴C-methanol. One out of 48 spots was detected for each target and the positions of the two spots were found to be Identical, thus indicating the specific detection of ¹⁴C-assimilating microorganisms and a microorganism capable of assimilating both acetate and methanol.