Unusual seismic oscillations with a period of 10 seconds were observed over a wide area of southernwestern Japan. Several lines of evidence indicate that the origin of these 10s waves must be around the Aso volcano, located in the central part of Kyushu Island, Japan. Although a 10s period is unusually long for a volcanic tremor and no corresponding surface activity at the crater was reported by the local observatory, the volcanic origin of the tremor is indisputable. A further astonishing aspects of the observations is that there exists a strong correlation between the onset of 10s waves observed remotely and cessation of high-frequency (>1 Hz) volcanic tremors at the Aso volcano. The Aso volcano appears to be trembling with a 10s period without this having been noticed. Since seismic monitoring of volcanic activity is limited to high frequencies (above 1 Hz) at the most of the volcanoes in the world, long-period tremors of the type observed here might be a widespread phenomenon that has been overlooked. ¿ American Geophysical Union 1994