  We present ground-based high-precision observations of the transit of
WASP-17b using the multi-band photometer ULTRACAM on ESO's NTT in the context
of performing transmission spectrophotometry of this highly inflated exoplanet.
Our choice of filters (SDSS u', g' and r' bands) is designed to probe for the
presence of opacity sources in the upper atmosphere. We find evidence for a
wavelength dependence in the planet radius in the form of enhanced absorption
in the SDSS r' band, consistent with a previously detected broad sodium
feature. We present a new independent measurement of the planetary radius at
Rpl = 1.97 +/- 0.06 Rjup, which confirms this planet as the most inflated
exoplanet known to date. Our measurements are most consistent with an
atmospheric profile devoid of enhanced TiO opacity, previously predicted to be
present for this planet.
