Tissue specimens from the mucosa of the pyloric gland area were subjected to electron microscopic investigation in 9 fasted male albino mice. Adequate preservation was achieved by fixing with osmium tetroxide at pH 8.5 and embedding in Vestopal W. The great majority of the gland cells in the pyloric mucosa are so-called pylorus cells. On the apical surface these cells are provided with a small number of microvilli. The lateral cell surfaces form numerous interdigitations with neighboring cells. The intercellular space often exhibits dilatations of different sizes. In the apical cytoplasm a varying number of closely packed secretory granules are observed. Their average diameter is about 0.32 μ, and their content is of moderate density. The mitochondria, which have a minor axis of about 0.23 μ, sometimes contain one or two granules enclosed by the dense layers of mitochondrial inner membranes. The diameter of these granules averages 900 Å. The Golgi apparatus is found in the paranuclear region and consists of flat vesicles and vacuoles. In its neighborhood secretory granules are often observed. α-Cytomembranes occur sparsely, but the free ribosomes are very numerous in the cytoplasm. Vacuole-containing bodies, dense microbodies, and fibrillar elements are other organelles in the pylorus cell cytoplasm. The nucleus is situated basally in the cell. Minor differences are observed between pylorus cells from different levels of the mucosa. Since gradual transformations occur between the different forms, there is no reason for any subdivision of the pylorus cells into different groups. Ultrastructurally the pylorus cells are indistinguishable from the mucoid fundus gland cells. Argyrophile cells are sometimes observed in the pyloric mucosa. Their ultrastructure agrees with those in other parts of the gastric mucosa. Zymogen cells and parietal cells are very rare in the pyloric gland area of the mouse.