The first microwave measurements of an electronically excited molecular species in the Earth's atmosphere are presented. Local thermodynamic equilibrium (LTE) rotational line emission from mesospheric O2(1Δg) was observed at a frequency of 255.01794 GHz (λ∼1.2 mm), employing the National Radio Astronomy Observatory (NRAO) millimeter facility at Kitt Peak, Arizona (32°N, 111°W). The pressure-broadened line shapes of the O2(1Δg) spectra, which were obtained in January and April 1992 and in January and November 1993, are inverted to retrieve O2(1Δg) mixing profiles over the 50–70 km altitude region. The observed daytime abundances exceed ozone abundances in the lower mesosphere, which are separately retrieved with coincident O3 spectral line (249.7886 GHz) observations. The January and November 1993 observations are binned into 20–60 min time intervals to study O2(1Δg) diurnal behavior. Derived abundances of O2(1Δg) between 50 and 70 km for the four observation dates are 9%, 31%, 3%, and 26%, respectively, each ±10% higher than predicted, based on the simple photochemistry of lower mesospheric O2(1Δg). Modeled variation of [O2(1Δg)] with time of day agrees with observed variation in that the observed difference between model and data abundances is constant throughout the daylight hours of each observation date. Model underprediction of [O2(1Δg)] is consistent with similar model underprediction of mesospheric [O3]. A perturbation to the photochemical model that forces decreased ozone chemical loss brings brings both model [O3] and [O2(1Δg)] into agreement with the observations. O2(1Δg) abundances derived from these 1.2 mm observations agree with [O2(1Δg)] values derived from comparable SME observations of the 1.27 μm emission, with assumption of a 3880 s O2(1Δg) radiative lifetime [Badger et al., 1965]. The 6800 s O2(1Δg) radiative lifetime proposed by Mlynczak and Nesbitt [1995] is ruled out by the similar comparison.