A new heteronemertean is described, Gastropion sanmatiensis n. gen. et n. sp. from Caleta de Los Loros, San Matias Gulf, Argentine Republic (41degree S; 64degree 05' W). G. sanmatiensis is mainly defined by: (1) the structure of its proboscis sheath;- (2) enteronephridia and exonephridia situated in the anterior part of the oesophagus; (3) a sphincter between the oesophagus and the intestine; (4) an intestinal sphincter. This lineid heteronemertean seems to be very well adapted to oesophageal respiration and excretion.