An experimental study was performed with the aim of investigating the structure of transitional and turbulent nonpremixed jet flames under different gravity conditions. Experiments were conducted under three gravity levels, viz., 1 g, 20 mg, and 100 mug. The milligravity and microgravity conditions were achieved by dropping a jet-flame rig in the University of Texas at Austin 1.25-s and NASA-Glenn Research Center 2.2-s drop towers, respectively. The flames studied were piloted nonpremixed propane, ethylene, and methane jet flames at source Reynolds numbers ranging from 2000 to 10,500. The principal diagnostic employed was time-resolved cinematographic imaging of the visible soot luminosity. Mean and root-mean-square (RMS) images were computed, and volume rendering of the image sequences was used to investigate the large-scale structure evolution and flame tip dynamics. The relative importance of buoyancy was quantified with the parameter, xi(L), as defined by Becker and Yamazaki (Combust. Flame 33 (1978) 123-149). The results showed, in contrast to some previous microgravity studies, that the high-Reynolds-number flames have the same flame length irrespective of the gravity level. The mean and RMS luminosity images and the volume renderings indicate that the large-scale structure and flame tip dynamics are essentially identical to those of purely momentum-driven flames provided xi(L) is less than approximately 2-3. The volume renderings show that the luminous structure velocities (i.e., celerities) normalized by the jet exit velocity are approximately constant for xi(L) < 6, but scale as xi(L)(3/2) for xi(L) > 8. The flame length fluctuation measurements and volume renderings also indicate that the luminous structures are more organized in low gravity than in normal gravity. Finally, taken as a whole, this study shows that xi(L) is a sufficient parameter for quantifying the effects of buoyancy on the fluctuating and mean characteristics of turbulent jet flames. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.