About 1.750 specimens of fossil Lepidoptera were found in marine sediments from the Paleocene/Eocene of Denmark. This is the most extensive fossil record of Lepidoptera we know so far. The great abundance indicates that moths and butterflies constituted a dominant insect group already about 55 million years ago. More than 1.000 specimens belong to one species of the Heteroneura. Commonly several individuals of this species are embedded closely together in the sediment. The great abundance of a single lepidopteran species in sediments of the former North Sea indicates mass migrations over the open sea. Beside the report of these material aspects of the fossilization of Lepidoptera and their flights over the sea are discussed.