A rare outbreak of hemlock sawfly, Neodiprion tsugae Middleton, was detected in August in drainages near the Canadian border in Idaho infesting several thousand acres. Some stands of western hemlock and subalpine fir were severely defoliated. Understory Engelmann spruce trees were lightly defoliated. The sawfly overwinters in the egg stage within current needles. Very few eggs could be found during a survey in October. Some hemlock trees may be topkilled, but natural control agents usually cause these sawfly outbreaks to collapse in 1 or 2 years. No control action was recommended.