Photoluminescence ~PL! kinetics of long-lifetime indirect excitons in a GaAs/Al xGa12xAs coupled quantum well characterized by a small in-plane random potential was studied at temperatures 1.5<T<15 K for a wide range of exciton densities. Strong deviations of the indirect exciton PL kinetics from monoexponential PL rise/decay were observed at low temperatures and high exciton densities. In particular, right after the excitation is switched off, the spectrally integrated indirect exciton PL intensity increased sharply. Simultaneously, the indirect exciton energy distribution was observed to narrow significantly. The observed increase in intensity is attributed to the sharp increase of occupation of the optically active exciton states. The energy distribution narrowing is explained in terms of the phonon mediated exciton energy relaxation in momentum space and in the in-plane random potential. @S0163-1829~99!00104-6# A number of collective phenomena, in particular exciton condensation and superfluidity, have been predicted for a low-temperature two-dimensional ~2D! exciton system in semiconductor quantum well structures. 1 The principal limitation for the realization of the low-temperature exciton system is the electron hole recombination, which limits the exciton lifetime and results in the high effective exciton temperature. The latter is determined by the ratio between the exciton energy relaxation rate and the exciton recombination rate. The indirect ~interwell! excitons in coupled quantum wells ~CQW’s! are characterized by a long recombination lifetime; therefore, CQW’s present an opportunity for experimental study of a high-density low-temperature exciton system. In the general case of nonresonant excitation, the occupation of low-energy exciton states that are responsible for the formation of collective states 2,3 is determined by the exciton energy relaxation and can be revealed in the exciton photoluminescence ~PL! kinetics. In the present paper we study the PL kinetics of long-lifetime indirect excitons in a GaAs/Al x Ga 12x As CQW characterized by a small in-plane random potential. The earlier studies of the PL kinetics of indirect excitons in GaAs/AlxGa12xAs CQW’s have demonstrated a long indirect exciton recombination time, 4‐7 revealed a metastable exciton energy distribution, 8 and allowed for the measurement of the exciton energy relaxation in the hopping regime. 9