Exclusion statistics in conformal field theory and the UCPF for WZW models

In this paper we further elaborate on the notion of fractional exclusion st atistics, as introduced by Haldane, in two-dimensional conformal field theo ry, and its connection to the Universal Chiral Partition Function as define d by McCoy and collaborators. We will argue that in general, besides the ps eudo-particles introduced recently by Guruswamy and Schoutens, one needs ad ditional 'null quasi-particles' to account for the null-states in the quasi -particle Fock space. We illustrate this in several examples of WZW-models. (C) 2000 Elsevier Science B.V. All rights reserved.