Quiescent oocytes of the monkey Cebus albifrons were examined with the electron microscope. In many respects the ultrastructure of these cells was similar to that of other mammalian species. Elongate and oval mitochondria, lamellar Golgi complexes, small profiles of smooth endoplasmic reticulum, and vacuolar organelles were randomly distributed around a round nucleus which usually contained a nucleolus and clumps of heterochromatin. Among the unusual morphological characteristics of these oocytes are 'membranous aggregates', membrane-bound organelles containing a complex of convoluted membranes, some very dense rod-like structures and a droplet of moderate density which resembles lipid. A similar droplet is frequently found in mitochondria. Rough endoplasmic reticulum is abundant in many of these oocytes, forming parallel arrays and concentric rings around the nucleus. Folded membrane complexes, apparent elaborations of smooth endoplasmic reticulum, are frequently found in the cytoplasm in continuity with cisternae of smooth and rough endoplasmic reticulum and associated with vesicles which often contain flocculent material. The morphology of Cebus oocytes suggests a greater rate of steroid and protein synthesis, transport, and storage than is usually indicated by the ultrastructure of other mammalian oocytes.