Abstract— Canine distemper caused by the paramyxovirus of the genus Morbilivirus is a highly contagious disease that mainly affects dogs of all ages, with an important socioeconomic impact on the Brazilian veterinary field due to the high cost of treatment, failures in vaccine immunization and the suffering of animals and owners. It presents itself in 2 forms, overacute, characterized by abrupt fever and sudden death and, acute, when the animals show signs of fever, prostration, lack of appetite, nasal and ocular secretions, conjunctivitis, vomiting, respiratory distress, anorexia, diarrhea, dehydration, and cutaneous rash, which may follow neurological symptoms such as paralysis, convulsions and death. From 2004 to 2015, approximately 622 samples from dogs with suspected distemper were sent to the Electron Microscopy Laboratory of the Instituto Biológico, São Paulo, SP, Brazil (oral and nasal swabs, feces, urine and organ fragments). for diagnosis of viral agents. The samples were processed using transmission electron microscopy (negative staining and resin embedding) and routine histopathological techniques. In the transmission electron microscope, 254 samples (40.83%) were visualized, pleomorphic, rounded or elongated, enveloped paramyxovirus particles containing helical herring-bone like nucleocapsid, measuring between 100 and 500 nm in diameter. In ultrathin sections of the brain, the presence of a nucleus with marginalized chromatin containing intranuclear inclusions was visualized. Intracytoplasmic granular amorphous inclusions, formed by viral nucleocapsids were also observed. Complete particles measuring 100 to 250 nm in diameter and incomplete particles, measuring on average 70 nm in diameter, budding from the plasma membrane could also be identified. Large areas of demyelination were also observed. Histological sections of the brain showed perivascular and focal mono and polyclonal encephalitis, monoclonal meningitis, congested blood vessels and parenchyma with areas of demyelination. The presence of eosinophilic inclusion bodies was also observed in several nerve cells (Corpuscles of Lentz).