Control of seed size involves complex interactions among the zygotic embryo and endosperm, the maternally derived seed coat, and the parent plant. Here we describe a mutant in Arabidopsis, megaintegumenta (mnt), in which seed size and weight are dramatically increased. One factor in this is extra cell division in the integuments surrounding mnt mutant ovules, leading to the formation of enlarged seed coats. Unusually for integument mutants, mnt does not impair female fertility. The mnt lesion also has pleiotropic effects on vegetative and floral development, causing extra cell division and expansion in many organs. mnt was identified as a mutant allele of AUXIN RESPONSE FACTOR 2 (ARF2), a member of a family of transcription factors that mediate gene expression in response to auxin. The mutant phenotype and gene expression studies described here provide evidence that MNT/ARF2 is a repressor of cell division and organ growth. The mutant phenotype also illustrates the importance of growth of the ovule before fertilization in determining final size of the seed.