A simple Omegatron mass analyzer used for measuring density and temperature of nitrogen in the 100 to 350 km region of the upper atmosphere has been developed. The mechanical and electrical configurations have been designed for rocket flight application, and the operating parameters optimized for the upper atmosphere measurement. This Omegatron is calibrated and flown as part of a sounding rocket experiment known as the Thermosphere Probe, which also contains an electron temperature probe for determining electron temperature and density. Several successful flights have shown that the Omegatron is a reliable device for this application.