We present high-speed multicolour photometry of the faint subdwarf B star SDSS J171722.08+58055.8 (m(B) = 16.7mag), which was recently discovered to be pulsating. The data were obtained during two consecutive nights in 2004 August using the three-channel photometer ULTRACAM attached to the 4.2 m William Herschel Telescope. We confirm the star to be oscillating and we refine the dominant frequency to 6.960 mHz. A second new frequency of 7.267 mHz was discovered. Both frequencies are significant in all three colors at level > 5 sigma and vary in phase in the three colors. We attempted mode identification for the strongest mode from its amplitude ratios but did not succeed.