The structural organization of mature root nodules ofMedicago salivaL. is studied by thin-sectioning, scanning, and freeze-fracturing techniques. The nodules are club-shaped, with their meristematic tissue near the tip of each nodule. The bacteroidal cells situated closer to the tip area are young, whereas those located closer to the neck where the nodule and root join are older. The shape of the bacteroids changes as they grow older. The bacteroids evolve gradually from uniform long-rod or long-club shapes into short-club, short-rod. pear-shaped, ellipsoid, spherical, and Y-shapes, which in turn evolve into pear and spherical shapes. During the early part of the bacteroid's life, the bacteroid is enclosed in a membrane envelope. In older bacteroidal cells, it was observed that a few membrane envelopes contained more than one bacteroid. In senescent bacteroidal cells, the membrane envelopes have disintegrated and dissolved. The plasma membranes of mature bacteroidal cells have high endo- and exo-cytotic activities relative to nonrhizobial-infected root nodule cells or newly infected bacteroidal cells. Endo- and exo-cytotic activities are also evident on the membrane envelopes of bacteroids. The plasma membrane of the bacteroids appears to have no endo- and exo-cytotic activity, for the vesicle-like structures observed on the plasma membrane in thin-sectioning and freeze-fracturing preparations are in fact constricted invaginations of the plasma membrane of the bacteroids, somewhat resembling the cristae of mitochondria.