Premise of the Study Bilateral symmetry in core eudicot flowers is established by the differential expression of CYCLOIDEA ( CYC ), DICHOTOMA ( DICH ), and RADIALIS ( RAD ), which are restricted to the dorsal portion of the flower, and DIVARICATA ( DIV ) , restricted to the ventral and lateral petals. Little is known regarding the evolution of these gene lineages in non‐core eudicots, and there are no reports on gene expression that can be used to assess whether the network predates the diversification of core eudicots. Methods Homologs of the RAD and DIV lineages were isolated from available genomes and transcriptomes, including those of three selected non‐core eudicot species, the magnoliid Aristolochia fimbriata and the monocots Cattleya trianae and Hypoxis decumbens . Phylogenetic analyses for each gene lineage were performed. RT ‐ PCR was used to evaluate the expression and putative contribution to floral symmetry in dissected floral organs of the selected species. Key Results RAD ‐like genes have undergone at least two duplication events before eudicot diversification, three before monocots and at least four in Orchidaceae. DIV ‐like genes also duplicated twice before eudicot diversification and underwent independent duplications specific to Orchidaceae. RAD ‐like and DIV ‐like genes have differential dorsiventral expression only in C. trianae , which contrasts with the homogeneous expression in the perianth of A. fimbriata . Conclusions Our results point to a common genetic regulatory network for floral symmetry in monocots and core eudicots, while alternative genetic mechanisms are likely driving the bilateral perianth symmetry in the early‐diverging angiosperm Aristolochia .