In Arabidopsis, transcripts of the floral organ identity genes APETALA3, APETALA1, AGAMOUS, and PISTILLATA are expressed only in a subset of the floral organs. To test and extend present models for establishment of floral organ identity, we constructed a transgenic line that expresses the AGAMOUS gene under the control of the APETALA3 promotor (pAP3::AG). In wild-type flowers, AGAMOUS is expressed in the third and fourth floral whorls, whereas APETALA3 is expressed at high levels in whorls two and three. Under the control of the APETALA3 promotor, AGAMOUS is misexpressed in the second whorl of the flower. The organs in the second whorl of pAP3::AG flowers either fail to develop or develop as stamens. When pAP3::AG is crossed to the homeotic mutants apetala1, apetala3, pistilla, agamous and superman, novel floral phenotypes result. In pAP3::AG agamous-3 mutants, the flowers are indeterminate and consist of a repetition of sepals and stamens, rather that sepals and petals, as is observed in agamous single mutants; the indeterminacy demonstrates that AG is required in the fourth whorl to make a flower determinate.