Developmental ultrastructure of late embryos and cotylocidium larval morphogenesis of Rohdella amazonica, an aspidogastrean parasite of fish, were studied to reveal the functional aspects of larvigenesis within the egg as well as phylogenetically relevant characteristics of the embryos and larvae in this basal trematode group. Gravid worms were removed from the intestine of naturally infected banded puffer fish Colomesus psittacus, collected from the Bay of Marajó, Paracauari River (Pará, Brazil) and processed by standard methods of transmission electron microscopy (TEM) and cytochemistry. During late cleavage and rearrangement of the blastomeres, the vitelline syncytium that plays a role in eggshell formation and nutrient provision to the embryo completes its apoptotic degeneration as the embryonic mass grows substantially. Early larval morphogenesis involves cellular positioning that defines the anteroposterior polarity of the differentiating larva. Progressing through larvigenesis, the anterior end forms a muscular oral sucker surrounding the mouth, which leads inward into the pharynx and expanding digestive cavity. At the posterior end, a large disc forms as a precursor to the eventual ventral disc. The fully formed cotylocidium, still within the eggshell, is flexed ventrally, bringing the 2 poles into near juxtaposition. The neodermatan tegument with outwardly projecting small microvilli becomes fully formed, as myocytons, a protonephridial system, and 2 glandular regions occupy the body's interior. The ultrastructural features described here are very similar to those reported for Aspidogaster limacoides from fish and somewhat similar to those reported for Cotylogaster occidentalis from molluscs, but differ from the more diverse larvae of neodermatan taxa that have been studied more extensively.