A rarely occurring structure that is apparently a modification of endoplasmic reticulum was observed in the epithelial cells of the ascending colon of the mouse. The structure consists of a stack of 3 to 15 parallel cisterna-like elements separated by about 67 nm. The stacked cisternae usually are located adjacent to the basal end of the nucleus or in the region between nucleus and basal cell membrane. Top and bottom cisternae of many of the stacks have patent lumens and their outer membranes are lined by ribosomes. Most frequently, middle cisternae have attenuated lumens in their central regions. In some instances the width of the cisternal lumens is similar throughout. Closely apposed pairs of cisternae also occur. Some stacks have a concentric configuration. The intercisternal space contains tightly packed vesicles (38 nm) arranged in a hexagonal array. Many of the vesicles are connected to the membranes of the cisternae by stalk-like projections. The vesicles also occur between the nuclear envelope and the adjacent parallel cistenae. Mitochondria are situated close to each stack.