A theoretical analysis of molecular excitons in crystalline p-Nitro-Aniline (PNA) is presented. The approach is general and can be straightforwardly extended to the calculation of the exciton structure of other molecular crystals or aggregates. Based on the evaluation of transition density matrices (TDM), the method allows to easily classify the excited states of interacting molecules on the basis of those of the constituent molecules. Exciton couplings of selected dimers within the PNA crystal have been evaluated and the exciton dispersion has been determined. The experimental absorption spectrum of PNA powders can be analysed in details based on the exciton density of states. (C) 2010 Elsevier B.V. All rights reserved.