We describe in this report a novel class of mutants that should facilitate the identification of genes required for progression through the mitotic cell cycle during seed development in angiosperms. Three non-allelictitan(ttn) mutants with related but distinct phenotypes are characterized. The common feature among these mutants is that endosperm nuclei become greatly enlarged and highly polyploid. The mutant embryo is composed of a few giant cells inttn1, several small cells inttn2, and produces a normal plant inttn3. Condensed chromosomes arrested at prophase of mitosis are found in the free nuclear endosperm ofttn1andttn2seeds. Large mitotic figures with excessive numbers of chromosomes are visible inttn3endosperm. Thettn1mutation appears to disrupt cytoskeletal organization because endosperm nuclei fail to migrate to the chalazal end of the seed. How double fertilization leads to the establishment of distinct patterns of mitosis and cytokinesis in the embryo and endosperm is a central question in plant reproductive biology. Molecular isolation ofTITANgenes should help to answer this question, as well as related issues concerning cell cycle regulation, chromosome movement and endosperm identity in angiosperms.