  We report the first experimental observation of a long-wavelength hosing
modulation of a high-intensity laser pulse. Side-view images of the scattered
optical radiation at the fundamental wave-length of the laser reveal a
transverse oscillation of the laser pulse during its propagation through
underdense plasma. The wavelength of the oscillation \lambda_hosing depends on
the background plasma density n_e and scales as \lambda_hosing~n_e^-3/2.
Comparisons with an analytical model and 2-dimensional particle-in-cell
simulations reveal that this laser hosing can be induced by a spatio-temporal
asymmetry of the intensity distribution in the laser focus which can be caused
by a misalignment of the parabolic focussing mirror or of the diffraction
gratings in the pulse compressor.
