Natural Douglas-fir tussock moth,Orgyia pseudotsugata(McDunnough), egg masses, or laboratory-produced egg masses that were placed in the field, were used to determine the activity of egg parasites.Telenomus californicusAshmead is the dominant egg parasite throughout the range of the tussock moth in the United States. Percentage of eggs parasitized varied substantially, from 0 to 65%, among 10 study areas in Oregon, California, Idaho, and Arizona. Aggregation of parasite attack as measured by Taylor's power law did not change over the range of parasitization rates observed. Parasitization appeared to be positively related to the outbreak potential of xeric outbreak-prone versus mesic nonoutbreak sites, parasitization being nearly 3-fold as high on the mesic sites as on the xeric sites. We developed a regression equation that relates proportion of egg masses attacked to total parasitization of eggs among masses. The regression provides a rapid, labor-saving way to estimate percentage of parasitized eggs.