Methinks that you are only adding arcs with the ADDELEMENT calls... Even though options.digraph is FALSE, you still have to incorporate both (i,j) and (j,i) into the graph in order to represent the undirected edge {i,j}. Otherwise, everything seems ok.. -- Dr. Gordon Royle, http://www.csse.uwa.edu.au, gordon(AT)csse.uwa.edu.au --