For better understanding of the partial melting and melt segregation processes in the upper mantle, we have studied the layered structure of a peridotite mass in Hokkaido, Japan. The layering is characterized by wavy and oscillatory patterns both in terms of bulk rock and mineral chemistry. From the observed linear trends in bulk rock chemistry, we have concluded that such layering was caused by segregation of partial melt in the mantle. Considering the geological and geochemical data, a physical model of melt segregation in the gravity field is put forward. Because permeable flow is strongly coupled with solid creep and compaction, magma waves may be generated by a fluid dynamic oscillation in a partially molten, ascending mantle. A vertical chemical stratification may thus be obtained, starting from a homogeneous mantle. According to the hypothesis, the most depleted part, dunite, is a residue of partial melting, and the least depleted part, plagioclase lherzolite, represents a melt accumulation region. The partial melt involved was probably picritic in composition. ¿American Geophysical Union 1987