The surface pH of a Physarum plasmodium advancing on an agar plate with a definite antero-posterior polarity was measured with a small antimony pH combination electrode. There was a clear pH gradient over the plasmodium surface from the advancing front (pH 6.6 ± 0.1) to the rear (pH 5.6 ± 0.4), whereas the surface pH of freshly formed endoplasmic drops was 6.9 ± 0.3. In addition to this regional pH difference, an oscillation of pH was detected over the entire plasmodium surface. In the front zone this oscillation did not disappear even when a small area surrounding the electrode tip was disconnected from the rest of the plasmodium, but in the strand region the oscillation disappeared as soon as the strand was disconnected near the electrode tip from the rest of the plasmodium. It took 10-20 min before the oscillation began again in the disconnected strand. This shows that the active site of oscillation is only in the front region and that the rest of the plasmodium is somehow under the control of the front part.