Lidar observations of the mesospheric sodium layer, made at S¿o Jos¿ dos Campos (23¿S, 46¿W), show three distinct types of organized structure in the vertical distribution of sodium. Profiles averaged over several days show a smooth but asymmetrical distribution. A sequence of profiles for a given night normally shows a wavelike structure which descends through the layer with time. Very occasionally an extremely narrow peak is observed at a constant height for several hours. On one occasion a layer 2.5 km wide, with scale heights of 700 m on the bottomside and 900 m on the topside, was observed to persist at a constant height for three hours. It is concluded that such a layer could neither be produced by neutral density perturbations nor by photochemical processes and therefore indicates a source of sodium in the mesosphere.