Background Canine myocarditis can result from infection with bacteria Listeria monocytogenes.Case report A 14-week-old female entire Staffordshire Bull Terrier was assessed for lethargy, inappetence and dyspnoea. Radiographs and echocardiography revealed fluid within the pericardial space, a plaque of marked hyperechogenicity within the right ventricular free wall, marked right atrial dilation and myocardial systolic dysfunction. Histopathology of the myocardium was consistent with severe pyogranulomatous myocarditis, with gram stain revealing gram-positive bacilli, consistent with a Listerial infection. Bacterial culture of the myocardium yielded a light growth of Listeria monocytogenes. ConclusionTo the best of the authors' knowledge, this is the first case of canine myocarditis caused by Listeria monocytogenes to be described and should be included as a differential diagnosis of myocarditis. Further, it prompts the consideration of potential zoonotic risks for veterinarians treating dogs with pericardial effusions.