We have determined the vibrational distribution of emission from the O2(b1&Sgr;g+) state in the terrestrial nightglow. Previous nightglow studies have been limited to detecting emission from the v=0 level, but the high sensitivity and resolution available in sky spectra taken with the Keck I telescope and the associated High Resolution Echelle Spectrometer (HIRES) have made it possible to observe O2(b1&Sgr;g+) levels up to v=15. Apart from emission from v=0, which is intense above the atmosphere due to ineffective quenching, the vibrational-level-dependent intensity distribution is bimodal, with maxima at v=3--4 and v=12, and a deep minimum at v=8. Excluding the v=0 level, the total system intensity is of the order of 150 rayleighs. The O2(b1&Sgr;g+) vibrational levels seem to fall into two categories: v=0, 1 and v≥2. The intensity of emission from the latter group tends to follow the intensity of the OH Meinel system, while that of the former group does not. We consider the possibility that at least a portion of the v≥2 group derives from OH chemistry, and also discuss the possible role of sodium chemistry. ¿ 2000 American Geophysical Union