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Dynamics of the deformation band formation during creep in an Al-Mg alloy was investigated by simultaneous high-speed video recording of deformation bands with an extremely high time resolution of 40 µs and measurement of the concomitant acoustic emission. The results are consistent with the mechanism where the trigger of a strain step on the creep curve is the nucleation and rapid widening of a primary deformation band, generating an acoustic emission signal within one millisecond. Correlations between various stages of the deformation band formation, acoustic emission, and stress response are explored in detail. Notably, the stress drop rate and the amplitude of the acoustic emission signal are found to be linearly related. The mechanism of the acoustic emission associated with the initial rapid stage of the band development is discussed.