AbstractParacrystalline aggregates of microtubules were observed by electron microscopy in some cells of the anterior pituitary glands from ten untreated chinchillas (Chinchilla laniger). Longitudinal section of the paracrystals revealed regular arrays of electron‐opaque lines parallel to the long axis of the microtubules and in transverse sections the paracrystals showed an unusual checkerboard pattern of closely packed microtubules. However, the inner wall of each subunit was hexagonal in shap