An analysis is presented of planar magnetic structures (PMSs) detected in the solar wind by the Ulysses spacecraft between its launch in October 1990 and the end of 1998. In all, 667 such structures were found, with durations between 6 hours (the minimum in the search algorithm) and 66 hours. The total amount of magnetometer data found to represent planar periods accounted for 9% of all data returned from the instrument during 1990--1998. PMSs are found to be a common occurrence within corotating interaction regions. Around one fifth of PMSs can probably be accounted for by the passage of shocks through the solar wind, leading to alignment and amplification of preexisting features. Heliospheric current sheet crossings were found to be close to about 50% of PMSs. The results thus favor several previously proposed PMS formation mechanisms. After other causes of PMSs such as draping around the interplanetary counterparts of coronal mass ejecta are taken into account, there still remains a sizeable minority of events that have no obvious association with other solar wind features. It is suggested that some of these unexplained PMSs may have been caused by compressional processes such as draping around transient features and had remained in the solar wind when the signatures of the features responsible were no longer recognizable. ¿ 2000 American Geophysical Union