Clinical toxoplasmosis is commonly reported in the cat, with the most consistent findings being ocular, pulmonic, hepatic, neurological, gastrointestinal and muscular abnormalities. Myocarditis, whilst frequently identified at post-mortem examination, has not been identified ante-mortem. In immunocompromised humans, myocarditis associated with toxoplasmosis is not an uncommon complication. In such cases, lymphocytic myocardial infiltration can lead to depressed myocardial function, which can be associated with congestive heart failure, rhythm disturbances and pericardial effusions. In addition, myocardial failure has been reported in immunocompetent humans associated with active toxoplasmosis [Chandenier J, Jarry G, Nassif D, Douadi Y, Paris L, Thulliez P, Bourges-Petit E, Raccurt C (2000) Congestive heart failure and myocarditis after seroconversion for toxoplasmosis in two immunocompetent patients. European Journal of Clinical Microbiological Infectious Disease 19, 375-379]. Here we describe a cat with echocardiographic changes consistent with infiltrative or inflammatory disease, and elevated IgG and IgM titres to Toxoplasma gondii. There was resolution of these myocardial changes once the toxoplasmosis was treated.