Oligonucleotide probes targeting the small-subunit rRNA are commonly used to detect and quantify bacteria in natural environments. We developed a PCR-based approach that allows synthesis of oligonucleotide probes targeting a variable region in the 16S rRNA without prior knowledge of the target sequence. Analysis of all 16S rRNA gene sequences in the Ribosomal Database Project database revealed two universal primer regions bracketing a variable, population-specific region. The probe synthesis is based on a two-step PCR amplification of this variable region in the 16S rRNA gene by using three universal bacterial primers. First, a double-stranded product is generated, which then serves as template in a linear amplification. After each of these steps, products are bound to magnetic beads and the primers are detached through hydrolysis of a ribonucleotide at the 3' end of the primers. This ultimately produces a single-stranded oligonucleotide of about 30 bases corresponding to the target. As probes, the oligonucleotides are highly specific and could discriminate between nucleic acids from closely and distantly related bacterial strains, including different species of VIBRIO: The method will facilitate rapid generation of oligonucleotide probes for large-scale hybridization assays such as screening of clone libraries or strain collections, ribotyping microarrays, and in situ hybridization. An additional advantage of the method is that fluorescently or radioactively labeled nucleotides can be incorporated during the second amplification, yielding intensely labeled probes.