For Arabidopsis and Antirrhinum, the so-called ABC model has been developed, which postulates that the determination of floral organ primordia is controlled by the action of three classes of homeotic genes. A number of these ABC genes encode putative transcription factors with the MADS box DNA binding motif. This paper reports on the functional analysis of the petunia MADS box gene fbp1. The temporal and spatial expression of fbp1 has been investigated in detail in transgenic plants containing the beta-glucuronidase (GUS) reporter gene fused to an fbp1 promoter fragment. fbp1-driven GUS activity was specifically detected in emerging petal and stamen primordia, suggesting a function of fbp1 in the control of second and third floral whorl identity. To test this hypothesis, transgenic petunia plants were generated in which fbp1 expression was inhibited by a co-suppression approach. The flowers of such plants exhibited homeotic conversions of petals towards sepals and stamens towards carpels. Occasionally, the third whorl carpels are fused forming a pentalocular gynoecium. This dominant fbp1 mutation acted as a single Mendelian trait in genetic crosses. These results strongly indicate that fbp1 is a petunia class B homeotic gene which is required for the correct initiation and determination of petals and stamens.