A new laser interferometer for geophysical purposes is being operated in an underground laboratory in central Italy. A simple electro-optical technique has been used to limit the costs. The main characteristics of this instrument are: high sensitivity(&Dgr;l/l≈10−12), fast sampling rate (up to 6 kHz), large dynamic range (unbounded in principle), the capability of following strain rates as high as7×10−5 s−1, and good reliability (two failures in nearly two years). ©1997 American Institute of Physics.