We have developed a web-based key to the sand-dwelling dinoflagellates, using Lucid software. Lucid software builds matrix keys and these are considerably easier to use than dichotomous keys. The key can also be used using platform-independent XID software. The species included have been compiled from descriptions in the literature of all recorded taxa from sediment habitats. Images have been taken using LM and SEM of samples from several sites in Australia, the North Friesian Wadden Sea, North Sea, Germany, and La Réunion Island (France, Loic Ten-Hage)) in the Indian Ocean. The key calls upon the resources within micro*scope (http://www.mbl.edu/microscope), an co-operative internet net resource for microbiology managed by the Bay Paul Center, Marine Biological Laboratory, Woods Hole, USA. The web site uses a taxonomic structure to organize information, and is currently comprehensive for genera of algae, and each entry can be linked to images, web pages, and other web-based resources such as Genbank, the American Type Culture Collection, and literature databases. Each identifiable element in the key is able to call up these resources. The key can be accessed at http://www.mbl.edu/baypaul/microscope/general/page_01b.htm