Parasystole is an arrhythmia resulting from co-existence of two independent impulse-forming foci. Ventricular parasystole which arises from an ectopic focus situated in ventricle, is relatively common. On the contrary, well-documented cases of atrial parasystole are extremely rare, and since the first case of atrial parasystole was reported by Kaufmann and Rothberger, only 24 cases have hitherto been reported. In the present paper is described and discussed a case of atrial parasystole with exit block, fusion phenomenon and interpolation. Case Report: A 60-year-old man was admitted to the hospi-tal for upper jaw carcinoma. He had not complained of his heart condition, and therefore, had never received digitalis nor any antiarrhythmic drug. The blood pressure was 134/72 mmHg, and the only cardiac findings were a functional murmur of grade 2 at the apex and a rhythm disturbance which was diagnosed clinically as premature systole. The chest roentgenogram showed evidence of slightly emphysematous findings, but no infection. Urinalysis, blood count and blood chemistry were all normal. The 12-lead ECG showed no abnormalities but an atrial arrhythmia. Analysis of Electrocardiograms: The electrocardiograms shown in Fig.1-3 are a continuous strip of standard lead II. Shown in Fig.1 are two different types of P waves, viz., one is the type of normal sinus P wave which is rather small and low in amplitude (under 0.l mV), and the other (marked P'), is the type of ectopic P wave. The ectopic P waves are positive, wide, sharp and tall. The premature P' waves (P'2, and P'3 in A, B and E. P'2, and P'4 in C. P'1 and P'3 in D) are not accurately coupled with their preceding normal sinus P waves. The coupling (P-P' interval) shown in Fig.1 widely varies in a range of 0.41 sec to 1.18 sec. The intervals between the ectopic P (P') waves (the inter-ectopic intervals) are all in simple multiples of 49 to 54 in 1/100 sec, thereby indicating that these beats are related to each other. This combination of conspicuously varied coupling and simply related interectopic intervals is evidence of atrial parasystole. Almost all intervals between the ectopic P (P') waves and the following sinus P waves-the returning cycle-are longer (about 1, 20 sec) than the normal sinus cycle, but as far as P'3 in B-strip is concerned, the returning cycle is considerably shorter (0.62 sec) than the measureable sinus cycle (P-P) in the same record, and the interval of P-P'3-P (1, 12 sec) remains the same as the sinus cycle. This phenomenon is the interpolated atrial premature systole.