The diurnal variation of the (0,0) (1Δg - 3Σg-) band of O2 has been observed with a filter photometer flows at 30 km on three balloon flights. The photometer is described. No emission originating from below 30 km was detected and consequently resonance fluorescence cannot play a significant part in the dayglow excitation process. The noon dayglow intensity was 20 MR in October and February while the nightglow brightness was about 90 kR. Theoretical calculations based on the ozone photolysis theory are described and compared with the observed time variations. During the period when the atmosphere is fully illuminated the observed interrsity variations are well reproduced theoretically with a rate constant of about 3 × 10-19 cm3 sec-1 for quenching of O2(1Δg) by atmospheric particles and the ozone height distribution taken to be exponential above 40 km. In the evening twilight, for solar depressions greater than 5°, the observed interrsity exceeded that calculated by about 500 kR. This discrepancy appears to be related to enhancements in the O2(1Δ) concentrations above 70 km as observed in recent rocket flights. © 1969.