Six portable, high-gain, high-frequency seismographs were operated in Nevada and California for several weeks before and after the underground nuclear explosion Benham to assess the possibility that earthquakes at distances of tens of kilometers or more may be triggered by large underground explosions. A pronounced increase in earthquake activity in the vicinity of the shot point was observed immediately after the detonation and continued for more than a month after the explosion. No significant change in activity within 25 km of any of our instruments northeast of the Nevada Test Site was observed, and the activity in Death Valley recorded after the explosion did not indicate an important increase. These data imply that this particular explosion did not significantly affect the seismicity of the region studied. Throughout the period of observation the seismic activity northeast of the Nevada Test Site was low; an average of about one event per day was detected within about 25 km of each station. This suggests that the current tectonic activity of this part of Nevada is lower than that of western Nevada and of most other tectonically active regions where microearthquake studies have been made.