The Caenorhabditis elegans excretory cell extends tubular processes, called canals, along the basolateral surface of the epidermis. Mutations in the exc‐5 gene cause tubulocystic defects in this canal. Ultrastructural analysis suggests that exc‐5 is required for the proper placement of cytoskeletal elements at the apical epithelial surface. exc‐5 encodes a protein homologous to guanine nucleotide exchange factors and contains motif architecture similar to that of FGD1, which is responsible for faciogenital dysplasia. exc‐5 interacts genetically with mig‐2, which encodes Rho GTPase. These results suggest that EXC‐5 controls the structural organization of the excretory canal by regulating Rho family GTPase activities.