The nematode Caenorhabditis elegans is used extensively by scientists to study a wide variety of biological processes and is one of the most thoroughly characterized animals. Over the years, the community of C. elegans researchers has generated a wealth of information on the genetics, development, behaviour, and cellular and molecular biology of the worm. This body of data has grown even larger with the recent application of high throughput screening methodology to study gene function, expression and interactions. WormBase (http://www.wormbase.org) is the primary online source of biological data on C. elegans and related nematodes. Equipped with an assortment of powerful search tools, WormBase allows users to quickly extract a variety of information, including data on individual genes, DNA sequence, cell lineage and literature citations. As the database is well maintained and the functionalities constantly modified in response to evolving researcher needs, WormBase has become a vital component of the laboratories studying the worm and a model for other biological databases.