This project investigates the propagation of ultra-high energy cosmic raynuclei and protons. A simulation of the propagation subjecting the particlesto energy losses due to cosmological redshift as well as interactions with theextra-galactic background radiation seeks to find the initial conditions at thesource which give the best results on Earth compared to the observations ofthe Pierre Auger Observatory (PAO).The results agree with previous works of the same kind that a chemicalcomposition of medium-weight fits the observed air shower data best. Thestarting conditions which gave the best results for the air shower characteristics<Xmax> and RMS(Xmax) were dN/dE proportional to E^{-alpha} with alpha = 1.6 for an initial chemical composition of 25% nitrogen and 75% silicon. Other combinations of themedium-weight nuclei also yielded similar results.No starting conditions could accommodate both the observed dN/dE and theair shower data simultaneously. Other works indicate that this might beimproved by the implementation of extra-galactic magnetic fields, but it couldalso indicate that the error margins in the observed data are underestimated