We report detection and measurement of the millimeter wave difference frequency between two near laser lines at 1.152 &mgr;. The two spectral transitions, separated by 2.26 Å, oscillate in pure neon in a single laser device, producing about 150 &mgr;W total power in several longitudinal modes. The many resulting 51.3‐kmc beat frequencies have been studied by optical heterodyne techniques. A type of diode has been found which has enough microwave sensitivityandenough optical sensitivity to combine the optical detector and the microwave heterodyne functions in a single element. The preliminary value for the difference in frequency between the and 2S4→ 2P7transitions in neon at 150 mtorr is (51,360 ± 150) Mc. This value is not in agreement with the value calculated from Vol. I of Sitterly'sAtomic Energy Levels(ref. 12).