Spontaneous twinning is a widespread but infrequent phenomenon in higher plants. We describe here a mutant of Arabidopsis thaliana, twin, that yields an unusually high frequency of viable twin and occasional triplet seedlings. Supernumerary embryos of twin arise through a novel mechanism: transformation of cells within the suspensor, a differentiated structure established early in embryogenesis. Twin embryos develop in tandem within the seed, connected by intact segments of the suspensor. Transformed suspensor cells appear to duplicate the patterns of cell division and developmental pathways characteristic of zygotic embryogenesis. In addition to polyembryony, mutant embryos exhibit a number of developmental defects, including irregular patterns of cell division and abnormal morphology. The TWIN locus therefore appears to be required for normal development of the embryo proper as well as suppression of embryogenic potential in the suspensor. The development of viable secondary embryos in twin demonstrates that cells of the Arabidopsis suspensor can successfully establish embryonic polarity and complete the full spectrum of developmental programs normally restricted to the embryo proper. In addition, the twin phenotype indicates that disruption of a single genetic locus can result in the conversion of a single terminally differentiated cell type to an embryogenic state.