Foliage leaves and flower organs are essential lateral organs used for photosynthesis and sexual reproduction in plants. These highly varied organs make important contributions to the adaptive value and morphological diversity in angiosperms. The YABBY gene family is involved in the regulation of these lateral organs in Arabidopsis thaliana . The role of these genes throughout angiosperm evolution is so far not fully understood. This study examines the extent to which the function of YABBYs is conserved outside the core eudicots, by studying their function in the basal eudicot Eschscholzia californica (California poppy). Putative orthologs of YABBYs were isolated from E. californica and their relations to other YABBYs were verified through phylogenetic analyses. Five YABBY-like genes were isolated, EcYAB1 to EcYAB5 (collectively referred to as EcYABBYs). In Arabidopsis, AINTEGUMENTA (ANT) acts together with YABBYs in regulating leaf polarity and blade outgrowth. To investigate whether this interaction is present in E. californica, two ANT-like sequences that were previously identified in E. californica, EcANT1 and EcANT2, were analyzed. Spatial and temporal expression studies showed expression of these genes during leaf and flower development. EcYABBYs transcripts were confined to the abaxial side of leaves and leaflets. EcYABBYs expression during leaflet development became restricted to regions of outgrowing leaflets and was absent from sinus regions between leaflets. Expression of EcANTs was very similar but not identical to that observed for EcYABBYs. EcANTs expression during leaflet development mirrored that of EcYABBYs, while EcANTs were expressed in a non-polar pattern in the leaf primordium. Transcript accumulation of both EcYABBYs and EcANTs suggested a role of these genes in E. californica leaf and leaflet development. Analyses of gene knock-down E. californica plants, generated through virus-induced gene silencing, confirmed this hypothesis. Knock-down EcYABBY plants formed fewer leaflets than control plants. In addition, leaflet development was altered; leaflets were often radial and misplaced within the blade. Knock-down EcANT plants had slightly fewer leaflets compared to control plants. Combined down-regulation of EcYABBY and EcANT led to a slightly greater reduction in leaflet number than knock-down EcYABBY plants. This suggests that the interaction of YABBY and ANT existed in the ancestor of eudicots.