The response of the near field of a free, plane air jet (aspect ratio 44:1) to a controlled, sinusoidal perturbation was investigated by hot-wire measurements. The experiments were carried out at an exit excitation amplitude of 1·4 % for the Strouhal number range 0·15 ≤ StH ≤ 0·6 and the Reynolds-number range 8 × 103 ≤ ReH ≤ 3·1 × 104. The influence of the excitation, introduced with a loudspeaker attached to the jet settling chamber, on the mean and fluctuating velocity fields is much weaker than that in the circular jet. The amplitude and phase profiles of the fundamental, educed through phase-locked measurements, show that the induced symmetric mode remains symmetric as it travels downstream. The wave growth rate is much higher and the wavelength much smaller in the shear layer than on the centre-line of the jet. The wave fundamental attains its maximum amplitude at StH ≃ 0·18 on the jet centre-line and at StH ≃ 0·45 in the shear layer. The amplitude profiles of the fundamental in the shear layer agree quite well with the spatial stability theory of Michalke (1965b); however, the phase data do not agree well with the theoretical predictions. The growth rate and the disturbance wavenumber increase monotonically with the StH both in the shear layer and on the centre-line but tend to approach constant values at higher StH. The phase velocity data show that, in the lower Strouhal-number range, the plane jet acts as a non-dispersive waveguide.