In early diplotene oocytes of X. laevis mitochondria are not dispersed all over the cytoplasm but gathered in a well described mitochondrial mass. Tracing these organelles during active vitellogenesis shows that some of them are involved in the elaboration of a cortical layer at the vegetative hemisphere of the cell while others stay around the nucleus. The latter contribute to the transient formation of a mitochondrial crown throughout active mitochondriogenesis. Autoradiographic studies of thymidine incorporation into mt[mitochondrial]DNA suggest a differential participation of each organelle to the final population of a full-grown oocyte according to its position in the cytoplasm.