Relationship between Sahelian mesoscale convective systems and easterly waves is studied at various time and space scales during 5 rainy seasons (1st July--15th September 1990--1994). This study focuses on a sub-population of MCSs called organised convective systems (OCS) which account for most of the rainfall. Wave activity periods are detected by spectral analyses on the 700 hPa meridional wind component, information on the convective systems is derived from an automatic tracking algorithm and rainfall data of the EPSAT-Niger experiment. We observe a modulation of OCS cloud cover at synoptic-scale during easterly wave activity, with an increase of the cloud cover in and ahead of the trough but OCS rainfall efficiency is maximum behind of the wave trough. At seasonal scale, OCS number and cloud cover distributions are not significantly affected by easterly wave occurrences, which means that the latter are not directly associated to the interannual variability of rainfall.