THE Arabidopsis gene SUPERMAN (SUP) is necessary for the proper spatial development of reproductive floral tissues(1-3) Recessive mutations cause extra stamens to form interior to the normal third whorl stamens, at the expense of fourth whorl carpel development(1-3). The mutant phenotype is associated with the ectopic expression of the B function genes, AP3 and PI, in the altered floral region, closer to the centre of the flower than in the wild type(3), and ap3 sup and pi sup double mutants exhibit a phenotype similar to ap3 and pi single mutants. These findings led to SUP being interpreted as an upstream negative regulator of the B function organ-identity genes, acting in the fourth whorl(2,3), to establish a boundary between stamen and carpel whorls. Here we show, using molecular cloning and analysis, that it is expressed in the third whorl and acts to maintain this boundary in developing flowers. The putative SUPERMAN protein contains one zinc-finger and a region resembling a basic leucine zipper motif, suggesting a function in transcriptional regulation.