Dividing cells of Tetrahymena pyriformis were observed by transmission electron microscopy for signs of morphogenesis of cortical structures. The earliest stage of basal body development observed was of a short cylinder of nine single tubules connected by an internal cartwheel structure. This is set perpendicular to the mature basal body at its anterior proximal surface under the transverse microtubules and next to the basal microtubules. Sequential stages show that the single tubules become triplet tubules and that the "probasal bodies" then elongate and tilt toward the organism's surface while maintaining a constant distance of 75-100 mmicro with the "parent." The new basal body after it is fully extended contacts the pellicle, and then assumes a parallel orientation with and moves anterior to the parent basal body. The electron-opaque core in the lumen of the basal body and accessory structures around its outer proximal surface appear after the developing basal body has elongated. These accessory structures associating with their counterparts from other basal bodies and with the longitudinal microtubules may play a role in the final positioning of basal bodies and thus in the maintenance of cortical patterns. Observations on a second sequence of basal body formation suggest that the oral anlage arises by multiple duplication of somatic basal bodies.