The frequency-magnitude distribution of earthquakes, characterized using the b-value, is examined as a function of space beneath Mount St. Helens (1988--1996), and Mt. Spurr (1991--1995). At Mount St. Helens, two volumes of anomalously high b (b>1.3) can be observed at depths of 2.6--3.6 km below the crater floor and below 6.4 km. These anomalies coincide with (1) the depth of vesiculation of ascending magma, and (2) the suggested location of a magma chamber at Mount St. Helens. Study of Mt. Spurr reveals an area of high b-value (b≥1.3) at a depth of about 2.3--4.5 km below the crater floor of the active vent Crater Peak. We propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b-value at shallow depths. Alternatively, interaction of magma with groundwater may have increased pore pressure and lowered the effective stress. The deeper anomaly at Mount St. Helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber. Our results indicate that detailed mapping of the frequency-magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers.¿ 1997 American Geophysical Union