Acoustic emission generated during the uniaxial compression of brittle rocks were counted after band-pass filtering. Throughout the whole process from application of load to failure, the count rate monitored through a low pass window was compared statistically with the monitored through a high passs window. The emission rate monitored through a low pas window increases more rapidly than that monitored through a high pass window as rock approaches failure. Two possible explanations for this effect are (1) generation of larger cracks (or coalescence of cracks into larger sizes) and (2) the relative attenuation of higher-frequency wave components. This finding is consistent with past observations, i.e., the relative number of large-amplitude events increases (i.e., a decrease in b value) as rock approaches failure. We suggest that larger cracks tend to generate events with larger amplitudes and containing lower-frequency components. The frequency characteristics of acoustic emissions are used to characterize the microfracturing processes leading to failure. The beginning of a sequence of acoustic emission activity is discussed in relation to the stress-strain curve and the onset of dilatancy. The present observations may be utilized in understanding structural instability of a region where rockbursts or earthquakes can potentially occur. If a sizable microseismic population is observed through sensors placed near the fault, then a potential exists for predicting major earthquakes.