In the retinal pigment epithelium (RPE) of a mammal, the Eastern gray squirrel, a type of cytoplasmic organelle, the so-called "myeloid body" that was previously thought to be restricted to the RPE of lower vertebrates was observed. In the squirrel, these organelles are continuous with the smooth endoplasmic reticulum (SER), lack an enclosing membrane, and in general exhibit all the morphologic criteria used to identify myeloid bodies. The presumptive myeloid bodies in the squirrel RPE are most prevalent in animals killed during the early hours of the dark period of a 12L:12D lighting cycle. They are rarely observed in animals killed just prior to or during the light period. Thus, these findings document for the first time the occurrence of myeloid bodies in the mammalian RPE, and indicate that their presence is influenced by a diurnal lighting cycle.