Bats are seasonal breeders and roost under a wide range of lighting conditions, from broad daylight to the total darkness of subterranean passageways and caves. Some are true hibernators. These characteristics and the paucity of information on their pineal organ prompted this investigation, which is based upon the pineals of 191 specimens of 88 species and 12 families of bats. Comparative morphological and volumetric observations have been made on serially sectioned brains of each species. Data include brain and body weights, mean pineal dimensions and volume, a computed pineal size index for each species and salient characteristics and relations of the pineal organ of the 12 chiropteran families. Generally speaking, despite some exceptions, larger bodied bats also have larger pineals. Bats of the microchiropteran families such as the Emballonuridae, Megadermatidae, Rhinolophidae, Hipposideridae, and a few vespertilionids (for example, Myotis adversus) and molossids (for example, Tadarida mops), have very large pineal organs, of which many reach the brain surface. All of these bat families inhabit dark caves. By contrast, in megachiropterans (pteropodids) which roost in broad daylight, the pineal lies deeply recessed and covered by the cerebral hemispheres. It is postulated that in general the superficial and deep location of pineal in micro- and megachiropteran species respectively may be a consequence of several factors, such as their habitat and their neocortical and cerebellar development. A system of classifying chiropteran pineal organs has been presented; in most species they are either of Type A or of Type AB. Most species have non-uniformly distributed parenchymal cells arranged in cords or clusters. In some species (for example, Rhinolopus trifoliatus and R. luctus) morphologically distinct dorsal and ventral divisions are observed. Pineal vascularity appears to be related to its size. Intrapineal neurons are rare and, when present, are associated with blood vessels. Epithelium-lined cavities are seen in the pineals of several species, while in a few others, the pineal is either absent or consists of a few scattered cells. Variable relationships between the pineal and the habenular commissure suggest that they may be unrelated functionally.