A comparison between one- and two-dimensional hybrid simulations of a collisionless quasi-parallel shock has been done in order to investigate the influence of the spatial dimensionality on the upstream and downstream spectra of the diffuse ions. The simulations are done for a shock Mach number of MA=4.6 and a magnetic field-shock normal angle of &THgr;Bn=10¿. Within the same time period (t=70 &OHgr;-1ci) the diffuse ions reach in the one-dimensional simulation, on average, higher energies than in the two-dimensional simulation. This manifests itself in a considerably flatter upstream spectrum at the high energy end. At lower energies the intensity in the 2-D case is an order of magnitude higher than in the 1-D case. The spectral shape exhibits in the 2-D case a more gradual transition from the solar wind thermal distribution to the diffuse distribution. In both simulations the particles drift about the same distance along the shock. The difference in spectra is due to a smaller motional electric field at the shock (in the shock frame) in the two-dimensional case as compared to the one-dimensional case. ¿ American Geophysical Union 1993