Abstract Summary: At advanced stages of working with user-defined protein and gene sequence collections, it is frequently necessary to link these data to the taxonomic tree and to extract subsets in accordance with taxonomic considerations. Since no general automatic tools had been available, this was a tedious manual effort. Our taxonomy workbench allows processing of sequence sets, mapping of these sets onto the taxonomic tree, collection of taxonomic subsets from them and printing of the whole tree or some part of it. As a side effect, the system enables queries to and navigation within the taxonomy database. Availability: An implementation of the taxonomy workbench is accessible for public use as a www-service at http://mendel.imp.univie.ac.at/taxonomy/. Software components for the command-line and for the www-version are available on request. Contact: Georg.Schneider@nt.imp.univie.ac.at; Frank.Eisenhaber@nt.imp.univie.ac.at Supplementary information: Documentation for the taxonomy workbench can be accessed at http://mendel.imp.univie.ac.at/taxonomy/help.html. * To whom correspondence should be addressed.