Abstract Endosperm, a storage tissue in the angiosperm seed, provides nutrients to the embryo during seed development and/or to the developing seedling during germination. A major event in endosperm development is the transition between the syncytial phase, during which the endosperm nuclei undergo many rounds of mitosis without cytokinesis, and the cellularized phase, during which cell walls form around the endosperm nuclei. The molecular processes controlling this phase transition are not understood. In agl62 seeds, the endosperm cellularizes prematurely, indicating that AGL62 is required for suppression of cellularization during the syncytial phase. AGL62 encodes a Type I MADS domain protein that likely functions as a transcription factor. During seed development, AGL62 is expressed exclusively in the endosperm. During wild-type endosperm development, AGL62 expression is strong during the syncytial phase and then declines abruptly just before cellularization. By contrast, in mutant seeds containing defects in some FERTILIZATION-INDEPENDENT SEED (FIS) class Polycomb group genes, the endosperm fails to cellularize and AGL62 expression fails to decline. Together, these data suggest that AGL62 suppresses cellularization during the syncytial phase of endosperm development and that endosperm cellularization is triggered via direct or indirect AGL62 inactivation by the FIS polycomb complex.