The class B genes, which belong to the MADS-box gene family, play important roles in regulating petal and stamen development in flowering plants. These genes exist in two different types termed DEFLCLENS (DEF)- and GLOBOSA (GLO)-like genes, and the B-function is provided by heterodimers of DEF- and GLO-like gene products. In order to understand the molecular mechanism of floral development in the agapanthaceous ornamental Agapanthus praecox ssp. orientalis, we produced and characterized transgenic A. praecox ssp. orientalis plants ectopically expressing the DEF- or GLCMike gene of the same plant, ApDEF or ApGLO. No visible morphological alterations were observed both in vegetative and floral organs of all the 7 independent transgenic plants containing ApDEF. On the other hand, in 4 out of 6 independent transgenic plants containing ApGLO, organs developed in whorl 4 showed noticeable morphological alteration: they were thick compared with carpels of non-transgenic plants, and had a branch tip. No apparent morphological alterations were observed in floral organs of the other 3 whorls. Scanning electron microscopic observations showed that the tip surface of whorl 4 organs of non-transgenic plants was covered with papilla cells, while there were few papilla cells on the tip surface of the morphologically altered whorl 4 organs of transgenic plants. In addition, epidermal cells of the morphologically altered whorl 4 organs of transgenic plants showed an intermediate morphology between those of ovaries and filaments of non-transgenic plants. Since it has been reported that ApDEF is expressed in all the 4 whorls of non-transgenic A. praecox ssp. orientalis, endogenous ApDEF products and transgenic ApGLO products may form heterodimers causing homeotic conversion in whorl 4 of transgenic plants.