An experimental study of detonation initiation in a stoichiometric propane-oxygen mixture by a high-voltage nanosecond gas discharge was performed in a detonation tube with a single-cell discharge chamber. The discharge study performed in this geometry showed that three modes of discharge development were realized under the experimental conditions: a spark mode with high-temperature channel formation, a streamer mode with nonuniform gas excitation, and a transient mode. Under spark and transient initiation, simultaneous ignition inside the discharge channel occurred, forming a shock wave and leading to a conventional deflagration-to-detonation transition (DDT) via an adiabatic explosion. The DDT length and time at I bar of initial pressure in the square smooth tube with a 20-mm transverse size amounted to 50 mm and 50 mu s, respectively. The streamer mode of discharge development at an initial pressure of I bar resulted in nonuniform mixture excitation and a successful DDT via a gradient mechanism, which was confirmed by high-speed time resolved ICCD imaging. The gradient mechanism implied a longer DDT time of 150 ps, a DDT run-up distance of 50 mm, and an initiation energy of I J, which is two orders of magnitude less than the direct initiation energy for a planar detonation under these conditions. (c) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.