Abstract: The changes of mitochondrial ultrastructure during vitellogenesis in oocytes of Bullacta exarata were investigated by transmission electron microscope method.The results show that mitochondria are the organelles that first involved in the information of yolk granules,their outer and inner morphology differ in different oocyte stages,and the numbers of typical ones decrease during the process of vitellogenesis.At the early stage of vitellogenesis,there are many mitochondria in ooplasm of the oocytes.These are rounded with numerous cristae and electron-dense matrix.Then some mitochondrial cristae and inner membrane begin to degenerate.At the mid-stage,the cristae and inner membrane disappear,there are a great number of small dense masses in the mitochondrial lumen,these masses grow in size and develop into mature yolk granules,many of these granules resemble mitochondria,and a number of transitional stages share both mitochondria and yolk body features.At the end stage of vitellogenesis,there are lots of yolk bodies increasing in the number and diameter,and there are some mitochondria among yolk bodies that supply energy to cell metabolism.Finally,the changes and functions of the mitochondria in vitellogenesis are discussed.