Fast photometric observations of a cool DAV star, GD 154, obtained with the Whole Earth Telescope are presented. GD 154 is one of the coolest pulsating DA white dwarfs and its study is relevant for understanding of the red edge of the ZZ Ceti instability strip. Its power spectrum is dominated by three independent modes (P_1=1186.5 s, P_2=1088.6 s and P_3=402.6 s). It also exhibits harmonics and linear combinations of these three modes. However, none of the half-integer harmonics, reported in previous observations, were present during the WET campaign. Assuming that the observed modes are trapped in the thin outer hydrogen layer, an identification of the pulsation modes is proposed. The rotation period (2.3 days) and the mass of the outer hydrogen layer (2 times 10(-10) M_{*} ) are derived.