A morphological study of the yolk nucleus in previtellogenic oocytes from A. bimacularus has been carried out. Six phases of development were described from its formation closed to the oocyte nucleus to its dispersion in the periferic ooplasm. Histochemical techniques indicate proteins, lipids and RNA in the yolk nucleus of A. bimaculatus, but there are no carbohydrate in this structure. The origin and functional activity of the teleost yolk nucleus are discussed.