  Extrasolar planetary transits are powerful tools to probe their atmosphere
and to extract key physical properties of planets, like their mean densities,
chemical compositions, or atmospheric structures. Every 3.5 days, the transits
of the gaseous planet orbiting HD209458 offer the opportunity to investigate
the spectral features of its atmosphere. We present here NUV transmission
spectroscopy of the transiting extrasolar planet HD209458b using HST/ACS. We
show the data analysis of the seven HST orbits which were used to observe two
transits of HD209458b. Due to various remaining systematics, the radius of the
planet in the NUV could not be extracted with a high precison. However, we
derived a radius of Rp=1.4Rjup+/-0.08 between 2800 and 3100 AA which is
consistent with previous measurements in the visible.
