We describe the synthesis and study of multicolor quenched autoligating (QUAL) probes for identification and discrimination of closely related RNA and DNA sequences in solution and in bacteria. In these probes, a dabsyl quencher doubles as an activator in the oligonucleotide-joining reaction. The oligonucleotides remain dark until they bind at adjacent sites, and "light up" on nucleophilic displacement of the dabsyl probe by the phosphorothioate probe. Four fluorescent dye conjugates were prepared and tested with probes and targets that differ by one nucleotide. Experiments on polymer beads show clear color-based discrimination of DNAs added in solution. Two-color quenched probe pairs were then tested in the discrimination of 16S rRNA sequences in Escherichia coli. Single nucleotide resolution was achieved in the cells with green/red QUAL probes, allowing identification of a one-base sequencing error in the 16S rRNA database. Finally, QUAL probes were successfully applied in live bacterial cells. The method requires only incubation followed by fluorescence imaging, and requires no enzymes, added reagents, cross-linking, fixing, or washes. Because probes must bind side-by-side to generate signal, there is little or no interference from unintended protein binding, which can occur with other probe types. The results suggest that QUAL probes may be of general use in the detection and identification of sequences in solution, on microarrays, and in microorganisms.