We report resonant pump and white probe femtosecond spectroscopy in the Al0.42Ga0.58As multivalley semiconductor. For this alloy composition the energy valleys are near the Gamma-X crossover. Our most interesting result refers to the differential transmission below gap which shows carrier induced absorption and a complex dynamics. The data can be modeled if we take into account the dependence of the gap renormalization on the occupation of each conduction band valley. We infer an interplay between direct gap renormalization and the ultrafast redistribution of the photoinjected carriers that follows ultrafast intervalley scattering in the scenario of multiple valleys close in energy in the conduction band. This shows on femtosecond timescale the contribution of the electron exchange interaction to the band gap narrowing in highly excited multivalley semiconductors. (C) 2002 Elsevier Science Ltd. All rights reserved.1214181185