Gnathostomulids are found mainly in medium or fine-grained sand mixed with detritus and are often very common in sand from seagrass beds. They may occur at high densities in anoxic, sulfide-rich conditions (population densities of more than 6000 individuals per liter of sediment have been reported). They occur from the intertidal zone to depths of hundreds of meters.

(Brusca and Brusca 2003; Sørensen and Sterrer 2009 and references therein; Margulis and Chapman 2010)