AbstractSolenofilomorphids are of world‐wide distribution and live in association with the sulfide system, the anoxic and micro‐oxic layers of marine sediments. The 58 species of turbellarians on an intertidal sand flat in North Carolina, U.S.A., formed three species assemblages. The solenofilomorphids are a major part of a sulfide system assemblage which is centered in the mid‐tide region and which is intermediate in density and diversity between the two separate species assemblages in the surface layers of the upper and lower tide regions. Oxygen avoidance does not explain restriction of species to the sulfide system, and food preferences for and behavioral responses to specific bacterial species are suggested as proximal causes for observed distribu