ABSTRACT: Motility and chemotactic responses to dissolved organic matter of bacteria living within different types of sediments were investigated down to 35 cm beneath the sediment surface. At least 20% of bacteria swim, although they often attach temporarily to solid surfaces, and they respond to point sources of organic matter. Altogether their motile behaviour and their relative densities are very similar to what has previously been found for planktonic bacteria. Bacterial motility in sand and silt is about half that of motility in moving water.