Immunogold labeling was used to study the distribution of (1-->3)-beta-glucans and (1-->3, 1-->4)-beta-glucans in the rice grain during cellularization of the endosperm. At approximately 3-5 d after pollination the syncytial endosperm is converted into a cellular tissue by three developmentally distinct types of wall. The initial free-growing anticlinal walls, which compartmentalize the syncytium into open-ended alveoli, are formed in the absence of mitosis and phragmoplasts. This stage is followed by unidirectional (centripetal) growth of the anticlinal walls mediated by adventitious phragmoplasts that form between adjacent interphase nuclei. Finally, the periclinal walls that divide the alveoli are formed in association with centripetally expanding interzonal phragmoplasts following karyokinesis. The second and third types of wall are formed alternately until the endosperm is cellular throughout. All three types of wall that cellularize the endosperm contain (1-->3)-beta-glucans but not (1-->3, 1-->4)-beta-glucans, whereas cell walls in the surrounding maternal tissues contain considerable amounts of (1-->3, 1-->4)-beta-glucans with (1-->3)-beta-glucans present only around plasmodesmata. The callosic endosperm walls remain thin and cell plate-like throughout the cellularization process, appearing to exhibit a prolonged juvenile state.