We have done frictional sliding experiments on Inada granite in double shear and monitored the acoustic emission (AE) produced and temporal changes in the microseismic b-value (where b is defined as the log-linear slope of the AE frequency-amplitude distribution), using both rough and smooth ground simulated fault surfaces. We have found, (i) the maximum amplitudes of AE events during stable sliding are strongly dependent on the surface roughness with smooth-ground surfaces giving smaller maximum AE amplitudes; (ii) b-values are related to the surface topographic fractal dimensions, so that in steady-state stable sliding smooth surfaces exhibit lower b-values than rough surfaces; (iii) the b-value falls before stick-slip instability. The change of b with slip we interpret in terms of evolving fractal crack damage during frictional sliding of the fault surfaces. ¿ 1998 American Geophysical Union