We present ground-based high-precision observations of the transit of WASP-17b using themultiband photometer ULTRACAM on ESO's New Technology Telescope (NTT) in thecontext of performing transmission spectrophotometry of this highly inflated exoplanet. Ourchoice of filters (SDSS u′, g′ and r′ bands) is designed to probe for the presence of opacitysources in the upper atmosphere. We find evidence for a wavelength dependence in the planetradius in the form of enhanced absorption in the SDSS r′ band, consistent with a previouslydetected broad sodium feature. We present a new independent measurement of the planetaryradius at Rpl = 1.97 ± 0.06RJ, which confirms this planet as the most inflated exoplanet knownto date. Our measurements are most consistent with an atmospheric profile devoid of enhancedTiO opacity, previously predicted to be present for this planet.