Evolution of diverse male tail epidermal features of representative species in the family Rhabditidae (Nematoda:Rhabditida) was mapped by parsimony on a molecular phylogeny inferred with nearly complete DNA sequences of small subunit ribosomal RNA genes. Although the molecular phylogeny is consistent with some previously proposed relationships, there are also some major differences, suggesting a revision of rhabditid taxonomy is required. To reconstruct male tail evolution, character states and homologies were determined with the aid of developmental profiling at the level of single cells. Because the model genetic system Caenorhabditis elegans is a member of Rhabditidae and allows the genetic and developmental mechanisms of morphogenesis to be elucidated, candidate genes and pathways can be proposed for several of the reconstructed evolutionary changes in male tail morphology.