The ultrastructural changes of the embryo sac of Petunia during and after fertilization are described. The pollen tube enters the embryo sac by growing through the filiform apparatus and discharges its content into one of the synergids (penetrated synergid). The volume of the penetrated synergid increases and the cell bursts at its chalazal pole. The synergid and discharged pollen tube cytoplasm merge and subsequently degenerate. The degeneration is marked by the darkening of the cytoplasm and the disappearance of organelles. A complex rough endoplasmic reticulum and numerous small spheres remain discernible longest. Two degenerating nuclei are present in the penetrated synergid. The ultrastructure of the zygote changes slightly during its early development. 50 hrs after pollination (10-15 hrs after fertilization) the nucleus does not contain chromatin clumps any more and is surrounded by a shell of plastids. The primary endosperm cell shows a number of marked changes after the formation of its nucleus. The ribosomes become aggregated into large polysomes and the plasma matrix becomes homogeneously electron-dense. Both the mitochondria and dictyosomes change in ultrastructure and shape. Plastids, without starch, appear as the endoplasmic reticulum becomes very extensive. This ultrastructural differentiation indicates a changing metabolic activity.