The cavity resonator used in this investigation is a silver-plated steel cylinder 6· 5 cm. in diameter and of adjustable length. Resonance in the H011 mode is established at a frequency in the region of 9000 Mc./sec., and the length is then varied to give successive resonances at half wave-length intervals. The wave-length is thus determined and this, together with the frequency, the diameter and a correction term involving the sharpness of resonance, enables the velocity to be calculated. This procedure has some advantage over that used previously by Essen & Gordon-Smith in which the measurements were made with a resonator of fixed dimensions. The wave-length is determined only from differences in length, the first resonant length not being used, and in this way certain end-effects, such as those due to the coupling loops and to surface imperfections, are eliminated or greatly reduced. Moreover, by using different frequencies, or different modes at the same frequency, the diameter can be eliminated from the calculations and a value of c thus obtained in terms of frequency and length both of which can be measured with high precision. The result obtained is 299, 792· 5 ± 3 km./sec., and is thus in close agreement with that obtained by Essen & Gordon-Smith with a fixed cavity and also with the value of c determined recently by Bergstrand with an optical method.