The Arabidopsis floral homeotic gene AGAMOUS (AG) is required for development of the reproductive organs (stamens and carpels). In ag mutants, the loss of AG function leads to the conversion of these organs to the perianth organs (petals and sepals). In contrast, mutations in another floral homeotic gene, APETALA2 (AP2), result in the replacement of the perianth organs by the reproductive organs. On the basis of these observations, it has been proposed that AG and AP2 act in an antagonistic fashion. To test this hypothesis, we have studied the effects of ectopically expressed AG in transgenic Arabidopsis plants. The flowers of the transgenic plants exhibit a range of phenotypes mirroring those of ap2 mutants. These experiments provide direct evidence of the proposed antagonism between AG and AP2 functions, and the results strongly suggest that AG does indeed inhibit AP2 function.