In August and September 1986, an earthquake swarm of possibly several hundred earthquakes occurred near Crested Butte, Colorado. The epicentral area is located within the Ruby Range in a region of extensive middle Tertiary volcanic and intrusive activity. The recording of this sequence has provided the best data to date to evaluate the source characteristics of an earthquake sequence in Colorado and the associated tectonic stresses. At least 200 events were recorded at regional distances; 30 events were Richter magnitude (ML) 1.6 and greater, and 16 were reported felt. The largest event, ML 3.5, occurred on 3 September. In addition to the regional recordings, a portable seismographic network was deployed from 19 to 26 August. Based on these data, 78 events were relocated using a master event technique. The earthquakes define a 6-km-long, northwest-striking planar zone dipping steeply to the northeast, between the depths of 2 and 11 km. Focal mechanisms indicate predominantly normal faulting with a minor left-lateral component on an approximately northwest-striking, northeast-dipping plane. These observations are all consistent with possible slip on the Treasure Mountain fault, a late Tertiary structure within the Ruby Range. The northeast-oriented T axes exhibited by the Crested Butte focal mechanisms are consistent with the regional extensional stress direction characteristic of the southern Rocky Mountains as indicated by other earthquake focal mechanisms. Within this extensional stress regime, earthquakes in western Colorado appear to be the result of normal slip on reactivated preexisting faults that are favorably oriented to the contemporary stress field.