The frequency-magnitude distribution of earthquakes measured by the b-value is mapped in 3 dimensions at Mount Pinatubo, Philippines, down to a depth of 6 km below the summit. We analyzed 1892 well-located earthquakes with magnitudes ML >=1.4, recorded from May 7 through August 7 1991, and used both the weighted least squares and maximum likelihood methods. With the weighted least squares method, we found that generally b-values are higher than normal (b=1.0), with values between b=1.1 and b=3.6. The computed b-values are lower in the areas adjacent to the vent and directly underneath it, whereas high b-values are found in patches located E-SE, and NW of the vent. Two prominent regions of anomalously high b-values (b>2.4) are resolved, the first is located roughly 3 km E-SE of the vent, at depths between 3 and 7 km; the second is located roughly 3 km NW of the summit, at depths between 3 and 8 km. There is also some indication of a deeper high b-value anomaly NE of the summit at depths between 12 and 16 km. The statistical differences between selected regions of low and high b-values are established at the 99% confidence level. The b-value results agree fairly well spatially with those derived from earlier P-wave tomographic studies. No significant changes in b-values were found as a function of time. We speculate that the high b-value anomalies around Mount Pinatubo are regions of increased crack density, or alternatively high pore pressure, related to the presence of magma bodies nearby.