Access to information in disparate databases via the World Wide Web is possible through the use of standard identifiers and Universal Resource Locator (URL) query syntax. The Genome Informatics Group has added links from the plant, animal and biological ACEDB databases at the National Agricultural Library to over two dozen genetic, molecular and bibliographic external databases. We also collaborate with external databases to establish reciprocal links. Examples of these external links and the markup rules that create them will be given for ACEDB version 4 databases. Several curatorial software tools to locate external identifiers for addition to their databases will also be described.