Several studies have been made of the minimum frequency excursions Δf required for detection of frequency modulation, with modulation frequency fm. [E. Zwicker, Acustica, 2, 125–133 (1952) R. H. Kay and D. R. Matthews, J. Physiol. 225, 657–677 (1972)]. The graph of minimum Δf vs fm has a pronounced peak at large values of fm. Further, the minimum Δf increases with decreasing signal intensity. The present paper shows that these results may be understood if fm detection at larger fm is regarded as the detection of the first lower sideband masked by the carrier. From available masking and modulation data it appears that fm sidebands beyond the first do not contribute to detection. To test this idea joint experiments on sine tone masking and fm detection were performed. In each case the masker frequency equaled the carrier frequency, fc of the modulated signal. The signal to be detected had frequency equal to the side band frequency, fc−fm. The results of the experiment support the masked-first-sideband model for fm detection at high modulation frequencies. [Work supported by NSF grant BNS 20225.]