Let G be a finite primitive permutation group and let κ(G) be the number of conjugacy classes of derangements in G. By a classical theorem of Jordan, κ(G)  1. In this paper, we classify the groups G with κ(G) = 1, and we use this to obtain new results on the structure of finite groups with an irreducible complex character that vanishes on a unique conjugacy class. We also obtain…