The nematode Ascolaimus elongatus (Bütschli, 1874) Ditlevsen, 1919 belongs to the marine nematodes (family: Axonolaimidae). It can be found in estuary and intertidal sand and mud in Europe and North-America. For a long time A. elongatus was the only known species within Ascolaimus, till 2010 when A. australis was described from Australia. Morphologically A. elongatus can be recognized by the presence of six small labial setae, four long cephalic setae and 4 rows of medium sized somatic setae within the pharynx region. It also has a relatively large amphid forming a rounded loop and a weakly sclerotized buccal cavity. The short nearly cylindrical tail includes three caudal glands typically with swollen ducts towards the tail tip. The male spiculus has distinct hooked distal ends and spade-shaped proximal cephalisation, the gubernaculum shows a relatively large dorso-caudal apophysis.