A species-specific DNA probe was developed to detect the dermatophyte species Trichophyton rubrum. The selected oligonucleotide sequence is derived from the highly variable internal transcribed spacer 2 region of the ribosomal DNA operon. The specificity of the non-radioactive labelled oligonucleotide probe was tested against related dermatophytes, other eukaryotic microorganisms and against human DNA. No cross-hybridization was found, and hybridization signals were invariably detected in all T. rubrum strains investigated. In addition, no homologous sequences were found searching the EMBL database. Experiments to establish a method for isolating DNA directly from clinical specimens gave successful amplification and hybridization products in about 30% of the samples.