The light-scattering coefficient of aerosols from biomass burning was measured as a function of relative humidity (RH) and wavelength in four regions of Brazil extending from the cerrado to the primary forests of the Amazon Basin. At a wavelength of 550 nm the ratio of the light-scattering coefficient at a RH of 80% to that at an RH of 30% ranged from 1.01 to 1.51, with an average value of 1.16. The aerosol backscatter ratio as a function of RH decreased by 10 to 20% as the RH was increased from 30 to 80%. Measurements of the geometric growth of the aerosol with RH were consistent with the measured changes in light scattering. Use of these new measurements yields an estimate for the globally averaged direct radiative forcing by biomass burning aerosol of about 0.25 W m-2. ¿ 1998 American Geophysical Union