Abstract Amaranthus hypochondriacus embryo sac development was investigated before and after fertilization. During the early stages of development, the young embryo sac displays three antipodal cells at the chalazal pole that degenerate very early in the maturation process, before the synergids and egg cell are completely differentiated. The mature embryo sac is composed only of the female germ unit. The synergid cells organize a filiform apparatus accompanied by the presence of mitochondria and dictyosomes with numerous vesicles. The involvement of the synergids in transport and secretory functions related to pollen tube attraction and guidance, are discussed. The egg cell is located at the micropylar pole near the synergids and displays exposed plasma membranes at the chalazal pole. The fertilized egg cell does not exhibit marked changes after fertilization except for the closure of the cell wall. The central cell is the largest cell of this very long embryo sac. The fused nucleus is close to the egg apparatus before fertilization and displays a remarkable chalazal migration after gamete delivery. The ultrastructure of the central cell cytoplasm and the numerous wall ingrowths around this cell suggest an important role in nutrient transportation. After gamete delivery, the embryo sac displays electron dense bodies that aggregate within the intercellular space between the synergids, egg cell and central cell. These bodies, that appear in the embryo sac of several plants, are probably involved in gamete delivery for double fertilization. The possibility of biparental inheritance of mitochondria in this plant is also discussed.