The sexual dimorphism of the corbulae of Aglaophenia tubulifera was studied by light- and scanning electron microscopy. In fertile male corbulae the ribs were unfused along their length and a free proximal rib absent. In females all ribs were fully fused except for a proximal free rib which grew underneath the corbula. Male A. lophocarpa from the Atlantic had unfused ribs too but in Mediterranean populations the ribs were partly fused with small gaps between them.