The developmental process that determines the arrangement of ray sensilla in the Caenorhabditis elegans male tail has been studied. It is shown that the adult arrangement of rays is determined by the placement of ray cells at specific sites in the epidermis of the last larval (L4) stage. Placement of ray cells at specific epidermal sites results from the generation of neurons and support cells in the epidermis near to their final positions, and the subsequent refinement of these positions by an active mechanism involving specific cellular associations. Positions of ray cells and adjacent epidermal cells have been studied during ray development by means of indirect immunofluorescence staining with an antibody to a cell junctional antigen. Mutations are described in six genes that alter the adult arrangement of the rays, frequently resulting in fusion of rays. Changes in the adult pattern of rays in mutants appear to result from prior changes in the epidermal positions of ray cells, and for two mutants it is suggested that this may be due to the inappropriate clustering of processes from neurons and support cells of adjacent rays. Development of the wild-type arrangement of rays appears to require the specification of molecular differences between the rays that affect the specificity of their cellular associations.