The stromatic migmatites of Nelaug (Tvedestrand area, Southern Norway) are investigated in detail. They show well developed layers of leucosomes, mesosomes and melanosomes. It is established that the mesosomes and leucosomes of these migmatites are different from each other texturally, mineralogically, and chemically. Also combinations of leucosome plus adjacent melanosome portions are chemically different from those of the mesosomes. These observations do not agree with the findings of Mehnert (1971) and do not fit into his genetic model.