During metamorphosis of the anadromous sea lamprey, Petromyzon marinus L., the larval intestine develops numerous longitudinal folds. The initiation of this event is marked by a decrease in diameter of the intestine and in the height of the typhlosole and is followed by an increase in numbers of cells undergoing DNA synthesis, first in the anterior intestine and later in the posterior intestine. Foci of proliferating cells along the epithelial walls of both regions result in the development of longitudinal folds which seem to elongate by further proliferation of the epithelial cells at their bases and by evagination of the underlying connective tissue and muscle. Slight variations in the mode of development of folds in the anterior and posterior intestines likely reflect the different functions of these regions in the adult. Metamorphosis of the lamprey provides an opportunity to study intestinal development in a vertebrate during a late stage of ontogeny.