Neosporosis was diagnosed in an American Staffordshire Terrier with collapse and arrhythmia episodes by serological and cardiovascular examination. Clinical signs, diagnosis and treatment of Neospora caninum are described. The dog did not develop any neurological abnormality, which is the most common manifestation. Furthermore, the dog was also serologically positive for antibodies to Toxoplasma gondii and negative for Bartonella henselae and Borrelia burgdorferi. Serological diagnosis was achieved by indirect immunofluorescence. The arrhythmia, cardiac markers, biochemistry, complete blood count and clinical signs gradually improved following therapy with clindamycin and antiarrhythmic therapy. After 3 months of therapy Toxoplasma gondii titres persisted low but Neospora caninum titres changed to negative values. According to the veterinary literature and to the authors’ knowledge this was one of the few cases successfully treated for this condition. These observations demonstrated that neosporosis should be considered in the differential diagnosis of myocarditis in dogs and can be successfully treated by supportive therapy and clindamycin.