In 1983, the U.S. Geological Survey instrumented Long Valley caldera, near Mammoth Lakes, California, with an array of borehole tiltmeters to detect the rapid deformation that might accompany incipient volcanic activity, such as the intrusion of a dike at shallow depth. The array consists of seven sites, within and surrounding the zone of the 1983 earthquake swarm. Five of these sites have instruments installed at a depth of 6 m and consist of two closely spaced installations, one containing a Rockwell/Kinemetrics instrument and the other an instrument designed by J. Westphal of the California Institute of Technology. The sixth site consists of two of Westphal's instruments and one Rockwell/Kinemetrics instrument, and the seventh site has two of the Rockwell/Kinemetrics instruments only, installed at a depth of 2 m. Since July 1985, all but one site have been equipped with a telemetry system that uses the Geostationary Operational environmental Satellite (GOES). Most sites are also equipped with a telemetry system based on telephone lines and radio links. This tiltmeter array affords unique opportunity to compare the relative merits of different instrument types and installation techniques under similar settings. In general, the results are similar for the two different instruments and installations, but local geologic conditions substantially affect the performance at each site. The records from the 6-m-deep boreholes have better stability and lower noise than those from the 2-m-deep boreholes. At one site, the tilt record appears to be dominated by perturbations related to local geothermal production. Performance at most other sites indicates that sudden deformation, with time spans shorter than about 1 week and amplitudes of about 5--10 μrad or greater, should easily be detected by the instruments within the array. ¿ American Geophysical Union 1987