During 2003 an unusual outbreak of the Gambel oak borer, Agrilus quercicola (Fisher), caused extensive damage to several species of oak in Denver, Araphaoe, and Douglas counties in Colorado. This followed several seasons of environmental stress that predisposed Gambel oak to the insect, but adults apparently dispersed more than 30 km during this outbreak to kill oaks a considerable distance from native Gambel oak stands. In choice tests, adults fed on foliage of Gambel oak, but not other herbaceous shrubs common in stands of Gambel oak. Yellow and neon yellow-green, but not white, sticky traps caught considerable numbers of adults. Adult flight activity began in late May, peaked in early summer, and ended in late August. A chalcid parasitoid, Phasgonophora sulcata Westwood, was noted to be an important natural enemy. No injury by this insect has been observed outside of native Gambel oak in years subsequent to the 2003 outbreak.