Ultrafast differential transmission measurements on a series of InN epilayers, grown by molecular beam epitaxy, have been employed to determine the carrier lifetimes and to probe the carrier recombination dynamics at room temperature. Differential transmission spectra reveal a peak energy of similar to0.7eV on these samples, supporting the existence of the narrow band gap of InN. In addition, we observed a fast initial hot carrier cooling followed by a slow recombination process after pulse excitation. Carrier lifetimes ranging from 48 ps to 1.3 ns have been measured in these samples. An inverse proportionality between the carrier lifetime and the free-electron concentration was observed, suggesting that the donor-like defects or impurities may stimulate a formation of non-radiative recombination centers, reducing the carrier lifetime. (C) 2004 Elsevier B.V. All rights reserved.