Oogenesis in the anuran Xenopus laevis can be divided into six stages based on the anatomy of the developing oocyte. Stage I consists of small (50 to 100 μ) colorless oocytes whose cytoplasm is transparent. Their large nuclei and mitochondrial masses are clearly visible in the intact oocyte. Stage II oocytes range up to 450 μ in diameter, and appear white and opaque. Stage I and II are both previtellogenic. Pigment synthesis and yolk accumulation (vitellogenesis) begins during Stage III. Vitellogenesis continues through Stage IV (600 to 1000 μ), the oocytes grow rapidly, and the animal and vegetal hemispheres become differentiated. By Stage V (1000 to 1200 μ) the oocytes have nearly reached their maximum size and yolk accumulation gradually ceases. Stage VI oocytes are characterized by the appearance of an essentially unpigmented equatorial band. They range in size from 1200 to 1300 μ, are postivtellogenic and ready for ovulation. These stages of oocyte development have been correlated with physiological and biochemical data related to oogenesis in Xenopus.