As a first step toward understanding their mechanism of morphological evolution, we compare the morphology and development of the male genitalia in 10 species of Rhabditidae, the family of nematodes that includes Caenorhabditis elegans. We describe a number of variable morphological characteristics and focus in particular on the differing arrangements of the caudal papillae or rays within the acellular fan. We analyze the development of the ray cells within the epidermis of the last larval stage and identify changes in cell positions and cell contacts that underlie evolutionary changes in the arrangement of the rays. Epidermal cell positions were determined by means of indirect immunofluorescence staining with a monoclonal antibody directed towards adherens junctions. Similarities between the species in the cellular arrangements during the earliest developmental stages allow us to propose homologies between the rays in different species. Evolutionary changes in the positions and order of homologous rays are correlated with shifts in cell positions during development. The results suggest that genes for cell recognition or adhesion proteins, or pattern formation genes that regulate cell recognition or adhesion proteins, may be important foci of evolutionary change affecting morphology.