An infinite family of biprimitive semisymmetric graphs

A regular and edge-transitive graph that is not vertex-transitive is said t o be semisymmetric. Every semisymmetric graph is necessarily bipartite, wit h the two parts having equal size and the automorphism group acting transit ively on each of these two parts. A semisymmetric graph is called biprimiti ve, ii its automorphism group acts primitively on each part. In this articl e, a classification of biprimitive semisymmetric graphs arising from the ac tion of the group PSL(2,p), p = +/-1(mod 8) a prime, acting on cosets of S- 4 is given, resulting in several new infinite families of biprimitive semis ymmetric graphs. (C) 1999 John Wiley & Sons, Inc.