The characterization of isolates of microbial organisms such as yeast or bacteria based on variations in the base sequence of their ribosomal RNA genes. It is used e.g. to identify organisms causing disease, for phylogenetic or geographical population studies, or to track the source of microbial contaminants in food. The DNA is extracted from a sample of a pure culture of the organism and digested by an appropriate restriction enzyme; the resulting restriction fragments are separated by gel electrophoresis and blotted onto a nylon membrane. A radiolabelled DNA probe is then added to bind to the ribosomal DNA fragments, thus revealing the pattern of ribosomal restriction fragments that is characteristic for the isolate. This ‘fingerprint’ can then be recorded, digitized, and stored in a database for analysis and comparison with other ribotypes.