The tissue polarity genes of Drosophila are required for correct establishment of planar polarity in epidermal structures, which in the eye is shown in the mirror-image symmetric arrangement of ommatidia relative to the dorsoventral midline. Mutations in the genes frizzled (fz), dishevelled (dsh) and prickle-spiny-legs (pk-sple) result in the loss of this mirror-image symmetry. fz encodes a serpentine receptor-like transmembrane protein required for reception and transmission of a polarity signal. Little else is known of the signalling pathway(s) involved other than that Dsh acts downstream of Fz. We have identified mutations in the Drosophila homologue of RhoA p21 GTPase, and by analysis of their phenotype show that RhoA is required for the generation of tissue polarity. Genetic interactions indicate a role for RhoA in signalling mediated by Fz and Dsh, and furthermore suggest that JNK/SAPK-like kinases are involved. These data are consistent with a Fz/RhoA signalling cascade analogous to the yeast pheromone signalling pathway and that proposed for activation of the serum response factor (SRF) in vertebrate cells.