SummaryTheArabidopsisfloral organ identity genesAPETALA3(AP3) andPISTILLATA(PI) encode related DNA‐binding proteins of the MADS family. Considerable evidence supports the hypothesis that a heterodimer of AP3 and PI is an essential component of B class activity. Allap3andpialleles characterized to date exhibit equivalent phenotypic defects in both whorls 2 and 3. In strongap3andpimutants, petals and stamens are missing and sepals and carpels develop in their place. Weakap3andpimutants exhibit partial conversions of petals to sepals and stamens to carpels. In this report, we describe the isolation and characterization ofpi‐5,an unusual B class mutant that exhibits defects in whorl 2 where sepals develop in place of petals, but third whorl stamens are most often normal.pi‐5flowers resemble those from 35S::SEP3 antisense plants.pi‐5contains missense mutation in the K domain (PIE125K). PIE125Kexhibits defects in heterodimerization with its partner protein AP3. Via a reverse yeast two‐hybrid screen, AP3K139Ewas isolated as a compensatory mutant of PIE125K. The compensatory interaction between PIE125Kand AP3K139Eis observed both in yeast two‐hybrid assays andin planta. On its own, AP3K139Eexhibits defects in specifying both petal and stamen identity. In addition, PIE125Kis defective in interaction with SEPALLATA proteins in both two‐ and three‐hybrid assays suggesting that PIE125Kis defective in forming higher order complexes of MADS proteins. The decreased concentration of PI/AP3/SEP complexes offers an explanation for the petal defects observed in bothpi‐5and 35S::SEP3 antisense plants.