In this paper, we study block transitive automorphism groups of 2-(v, k, 1) block designs. Let D be a 2-(v, k, 1) (k = 6, 7, 8, 9) design admitting a block transitive, point primitive but not flag transitive group G of automorphisms. We prove that if G is unsolvable, then G does not admit an exceptional simple group of Lie type as its socle. Moreover, for a 2-(v, 9, 1) design, we also prove that there does not exist any block transitive, point imprimitive, unsolvable group G of automorphisms. (C) 2007 Elsevier B.V. All rights reserved.