In conjunction with a proposed suppression project against the Douglas-fir tussock moth in northern Idaho, more than 1,000 larvae were collected from sample plots throughout the 32,000-acre treatment area. We had hoped to determine naturally occurring mortality factors which may have contributed to lower than anticipated population levels. Reared on artificial media, 71.5 percent of the larvae completed their development to the adult stage. Parasites accounted for 10.6 percent of the pre-adult mortality, native NPV only 3.1 percent; 12.9 percent died of other diseases or unknown causes. No single factor was identified as having resulted in unexpectedly low tussock moth populations in 1986.