It is described the histopathology of the infection of Tilapia rendalli
(Osteichthyes, Perciformes, Cichlidae) and Hypostomus regani
(Osteichthyes, Siluriformes, Loricariidae) by lasidium larvae of Anodontites trapesialis
(Mollusca, Bivalvia, Mycetopodidae). The larvae were encysted within the epidermis of the host, being surrounded by a thin hyaline membrane, 3-6 μm thick, of parasite origin. A proliferative host cell reaction did not occur. The histopathology of the infection shows that the lesions induced by the parasites are minimal. However, the numerous small lesions produced by the release of the larvae may provide optimal conditions for the infection by opportunistic pathogens, namely fungus, which may eventually cause the death of the host.