The object of the present study was to examine ovarian oocytes of the sciuromorph rodent. Thomomys townsendii, to determine if they contained cytoplasmic inclusions similar to those reported in myomorph rodents. Our observations indicated the presence of cytoplasmic lamellar inclusions of oocytes of primary, secondary and large vesicular follicles of Thomomys. Small primary oocytes contained numerous cytoplasmic lamellae which appeared as spirals or concentric rings in cross-section. Longitudinal sections suggested the inclusions were arranged as concentric cylindrical sheets of material. Secondary and vesicular oocytes had morphologically similar inclusions, but, in addition, often had a tubular element of the endoplasmic reticulum associated with the core of the lamellae. Tangential sections through the lamellae revealed a crystalline substructure, regardless of the age of the oocyte. Our results indicate that cytoplasmic lamellar inclusions are not restricted to myomorph rodents, but may well be present in other species. The lamellar inclusions are three-dimensionally most similar in structure to those described in the hamster, but several fine structural differences exist even between these species. Thomomys is the only species to date in which a close topographical association develops between the lamellar inclusions and elements of the endoplasmic reticulum. Since the lamellae-endoplasmic reticulum association was not observed in the youngest oocytes examined, it is unlikely the endoplasmic reticulum is involved in synthesis of the lamellae, but it may, nonetheless, be metabolically related to the lamellae.