RegulonDB version 2.0, a database on transcriptional regulation and operon organization in Escherichia coli, is now available on the web at the following URL: http://www.cifn.unam. mx/Computational_Biology/regulondb/. In this paper we describe the main computational changes to the database, which include migrating the database to Sybase, providing graphical descriptions of the internal organization of operons and regulons, and direct links to MEDLINE references. The web interface offers searching either by mechanisms of regulation or by operon organization. The results of a search (operon organization, or site collection) are displayed as hypertext, and can also be displayed graphically. In terms of its contents, RegulonDB contains a large number of operons, as well as the absolute position in the completed genome sequence of sites, promoters, and individual genes of E.coli.