We report Q-estimates from borehole-to-tunnel seismic experiments at Nagra's ''Grimsel Test Site'' (GTS). The probed granitic rocks are cut by brittle faults but do not show any signs of pervasive weathering. Q-values estimated from P-wave first arrivals by the spectral ratio and rise time methods range from about 20 to 60 with a median value of 35 and a standard error of 20% or less. The surprisingly low Q-values inferred indicate that even outside the weathering zone, seismic attenuation in some parts of the uppermost crystalline crust may be very high. ¿ American Geophysical Union 1996