This paper focusses on a particular FASTEX Intensive Observation Period: the FASTEX special soundings during that period are added to the conventional data set and assimilated with a 4D-Var scheme under development at Météo-France. Results show a consistent use of the high cadence FASTEX ship soundings by the 4D-Var formulation and an improvement brought by this last scheme over 3D-Var when using only the vertical profiles synchronous with the synoptic time. The application of the 4D-Var analysis to dropsondes launched by a system-relative flight designed by the Joint Centre for Mesoscale Meteorology also produces a fine scale description of sub-structures of the mature system.