Arabidopsis flowers develop from groups of undifferentiated cells on the flank of an inflorescence meristem. The cells in these flower primordia must somehow assess their position within the primordium and differentiate accordingly to produce a flower with a precisely defined pattern of organ types and positions. The molecular mechanisms by which this is accomplished are largely unknown. We are studying a set of genes whose mutations give homeotic phenotypes in Arabidopsis flowers. A genetic model to explain the specification of organ identity by combinatorial action of the products of these homeotic genes is presented, along with several aspects that are not readily addressed by the model. The recent cloning of one of the Arabidopsis homeotic genes, and an additional homeotic gene from Antirrhinum, has provided an opportunity for molecular tests of our genetic model. So far, the molecular data are in accord with the genetic model.