We mapped b-values in three dimensions in a cube of the crust with approximately 20 km dimensions under Mt. Etna. The earthquake catalog used contains 450 events with M≥2.5 for 1990--1997.9. Comparison of b-values derived from samples of N=50 events, extracted from volumes with radii ranging from 2.5 to 6 km, showed statistically highly significant differences between b=1.5¿0.3 in most of the crust and b=3.0¿0.5 in two volumes with r≤5 km, one located 2 km E of the summit of Etna and at 10¿3 km depth and the other one WSW of Etna at a depth of about 3¿2 km below sea level. We interpret these high b-value anomalies as an expression of two active magma chambers at those locations. ¿ 1999 American Geophysical Union