Observations of the atmospheric extinction of solar ultraviolet radiation in the 142- to 149-nm and 155- to 168-nm bands have been used to determine molecular oxygen densities over the 100- to 160-km altitude range in a series of rocket flights from Woomera, Australia (31¿S, 136.5¿E). Comparison of data from morning and evening flights made on the same day (October 2, 1974) indicates that the evening density in the 107-118 km region was approximately 1.5 times that observed in the morning. With the exception of the sole evening flight, all the measurements in the 100- to 120-km region are significantly below the mean Cira (1972) model. Observations made following a period of magnetic disturbance with Kp = 4+ indicate that the molecular oxygen density near 150 km was enhanced by a factor of about 1.5 compared with similar observations made under quiet conditions with Kp = 1-.