The larval stages of Anguillicola crassus, a parasite of the swimbladder of eels, are described, using light and scanning electron microscopy. Second-stage larvae in copepods are characterized by a globular cephalic end and third- and fourth-stage larvae by an apical selerotized ornamentation consisting of two U-shaped median pieces. Third- and fourth-stage larvae are difficult to distinguish from each other using light microscopy because the third moult has never been observed and there are no obvious characters differentiating the two stages. Three characters, however, observed by using high magnification of light and scanning electron microscopy are considered as distinctive between the two stages, namely presence or absence of deirids and lateral alae and differences in the structure of the cuticle. These morphological characters will permit epidemiological studies at the larval level.