AINTEGUMENTA (ANT) was previously shown to be involved in floral organ initiation and growth in Arabidopsis. ant flowers have fewer and smaller floral organs and possess ovules that lack integuments and a functional embryo sac. The present work shows that young floral meristems of ant plants are smaller than those in wild type. Failure to initiate the full number of organ primordia in ant flowers may result from insufficient numbers of meristematic cells. The decreased size of ant floral organs appears to be a consequence of decreased cell division within organ primordia. Ectopic expression of ANT under the control of the constitutive 35S promoter results in the development of larger floral organs. The number and shape of these organs is not altered and the size of vegetative organs is normal. Microscopic and molecular analyses indicate that the increased size of 35S::ANT sepals is the result of increased cell division, whereas the increased sizes of 35S::ANT petals, stamens, and carpels are primarily attributable to increased cell expansion. In addition, 35S::ANT ovules often exhibit increased growth of the nucellus and the funiculus. These results suggest that ANT stimulates cell growth in floral organs.