An analytical expression is obtained for the mass yield distribution in fission induced by high-energy protons (100–340 MeV) on238U. The reaction is treated within the context of a two-step model. In the first step, the proton hits the target, knocks out some of its nucleons, yielding a residual nucleus with a distribution of excitation energies. In the second step, a fraction of the excitation energy of the residual nucleus is exhausted in the emission of several nucleons before it fissions. The multiplicity of the pre-scission neutrons is assumed to follow a Poisson distribution. The mass yield distribution, resulting from the fission of the remnant nucleus, is described within the hypothesis of the two independent fission modes, with parameters extracted from the analysis of low-energy fission, assuming that post-scission neutron emission weakly depends on excitation energy. Comparison with experimental data allows us to extract values for the excitation energy at which shell effects disappear.