We have examined the geographic distribution and prevalence of insect-specific viral infections in Douglas-fir tussock moth populations in British Columbia. Nucleic acids extracted from field collected egg masses from 10 different locations in British Columbia were subjected to agarose gel electrophoresis and Southern blot analysis. Electrophoresis of nucleic acids after digestion with RNase A at high and low ionic strengths showed that the extra bands detected in agarose gels were double-stranded RNA (dsRNA). The dsRNA banding pattern was similar to the electropherotype of cytoplasmic polyhedrosis virus type 5. CPV infections were found in 27 of 170 DFTM larvae reared from field collected egg masses from four of 10 sites examined in three geographic regions of British Columbia. The CPV-infection rate for the DFTM larvae ranged from 10 to 86% depending on the location of the site.