An artificially-induced epizootic was created in European pine sawfly(N. senifer)populations at density levels of 5, 7, and 10 colonies per tree when plots were treated with 3.5 × 108polyhedral inclusion bodies (PIB) per 0.04 ha. The progress of the epizootic was followed daily. The onset of mortality was at 13 days posttreatment, with a peak mortality period between 14 and 16 days across all density situations. During this period, ca. 70% of the total larval mortality was recorded. Of all the larval instars that died, the 4th experienced the greatest percentage of virus-induced mortality (ca. 58%). The average number of PIB accumulated per larva was inversely related to the density situations. That is, larvae in a low-density situation (five colonies per tree) accumulated more virus than those in a moderate-density situation (seven colonies per tree), and similarly between the moderate and high-density situations (10 colonies per tree). This resulted in an inverse relationship between the average number of PIB produced per plot and colony density per tree: 2.92 × 109, 9.00 × 108, and 3.3 × 108, in low-, moderate-, and high-density situations, respectively.