We report two patients who had isolated atrial fibrillation in the left atrium and regular activation of the entire right atrium. Mapping of the arrhythmia was performed using a 64-electrode basket catheter that was inserted intravenously and deployed in the right and left atria. Both patients manifested a single, stable interatrial electrical connection conducting in a left-to-right direction, consistent with Bachmann's bundle location. The right and left sides of the interatrial septum were activated discordantly, each reflecting activation characteristics of the respective atria. A filtering effect at the level of interatrial septum was demonstrated by calculating the fibrillation intervals on both sides of the operative interatrial connection. It was concluded that differences in activation of the left and right surfaces of the interatrial septum and preferential use and the filtering effect of the interatrial connections play a significant role in explaining the differences in activation patterns of the left and right atria in patients with atrial fibrillation.