The polarization properties of the backscattering of a lidar pulse from atmospheric clouds have been measured. A linearly polarized signal from a ruby laser at 694.3 nm is transmitted, and the scattering is simultaneously measured on a three-channel receiver that has polarizers oriented parallel, perpendicular, and at 45 degrees to the transmitted polarization. Substantial depolarizations (up to 0.5) are observed, and from the spatial variation of this depolarization the contribution of multiple scattering can be deduced.A wide variation in polarization properties is observed in different cloud types, and the results indicate that polarization signatures could be useful for cloud characterization and classification.