A case of interatrial block is reported, involving the upper part of left atrium, demonstrated by oesophageal recordings at different levels. Abnormally retrograde atrial depolarisation in the upper part of the left atrium was observed, but normal descending depolarisation in the lower part of the left atrium suggested that interatrial conduction was blocked on the Bachmann bundle but was normal on the middle interatrial pathway. The surface ECG showed a P wave of normal duration; it was inverted in Lead III and a VF, and flat in Lead II with a normal PR interval. The author suggests a method of classification of interatrial block into three types.