Summary Several lines of evidence suggest that sterile floral organs, collectively known as the perianth, have evolved multiple times during the evolution of the angiosperms. In the family Aristolochiaceae, the perianth is formed by two whorls of organs in the genus Saruma but by only one whorl in the remaining genera, including Aristolochia. Although the morphology of Saruma is similar in appearance to the core eudicot perianth, with leaf-like sepals and showy colored petals, the unipartite perianth of Aristolochia combines morphological aspects of both calyx and corolla. To investigate the organ identity program functioning in the novel perianth of Aristolochia, we identified homologs of the B-class genes APETALA3 (AP3) and PISTILLATA (PI) in both Saruma and Aristolochia. The expression patterns of these genes in Saruma indicate they are functioning in the development of the second whorl petaloid organs and third whorl stamens. In Aristolochia, however, the expression of AP3 and PI homologs in the perianth does not suggest a role in organ identity but, rather, in promoting late aspects of cell differentiation. The implications of these findings for the evolution of both petaloidy and B gene function are discussed.