The biology of marine sediments is dominated by the activities of the microbial communities. Sands and muds are home to a rich and abundant biota of prokaryotes, ciliates, and various flagellates especially euglenids and dinoflagellates. Dinoflagellates may be common and abundant in shallow water sediments which are illuminated or have a good supply of organic material. The communities of dinoflagellates may include species known to produce environmental toxins. The species of dinoflagellates listed here as living within the sediments is believed to be comprehensive at the time of writing (March 2002). Most of the images shown here were taken from sites in the German Bight (illustrated) and Botany Bay (Australia). Image: Mona Hoppenrath. Image used with permission by MBL (micro*scope).