Observations of weak stimulated Brillouin scattering with the 50-MHz incoherent scatter radar at Jicamarca are described. The nonlinear interaction of the upgoing radar wave with the downcoming waves, incoherently backscattered from greater heights, causes an asymmetry in the incoherent ‘ion’ backscatter spectrum. For a given antenna aperture this asymmetry increases with increasing power and decreasing frequency of the radar. The theoretically predicted effect was unambiguously verified, using 3-ms pulses and long integration time. Model calculations based on the theory presented show good agreement with the observations.