The effect of aerial application of Bacillus thuringiensis variety kurstaki Berliner ( Btk) on mortality of gypsy moth larvae caused by the entomopathogens Entomophaga maimaiga Humber, Shimazu, & Soper, and the gypsy moth nucleopolyhedrosis virus ( LdNPV) was evaluated. Three sites in 1997 and four sites in 1998 were established in a randomized complete block design. Larvae were collected weekly to determine the proportion infected with E. maimaiga and LdNPV. In 1997, epizootics of E. maimaiga in control plots boosted infection rates (61%) to levels twice that in Btk-treated plots (33%). Postseason egg mass density and defoliation did not differ between treatments, indicating population reduction early in the season by Btk or late in the season by E. maimaiga was equivalent. Few larvae became infected with LdNPV in 1997, probably because of the relatively low densities of gypsy moth larvae. In 1998, E. maimaiga was again more active in control plots than in Btk-treated plots, but dry weather conditions suppressed activity of E. maimaiga in all plots compared with 1997. LdNPV was also more active in control plots than in Btk-treated plots. On the peak infection date, infection levels in control plots (26%) were sixfold higher than in Btk-treated plots (4%). Despite greater LdNPV activity in control plots in 1998, aerial application of Btk was much more effective than natural LdNPV in suppressing gypsy moth populations. Final egg mass counts were 89% lower in Btk-treated plots compared with control plots. E. maimaiga and LdNPV disease prevalence increased with increases in larval density. Early spring inoculum levels and weather conditions were similar for all plots. Therefore, reduced infection levels in Btk-treated plots were most likely the result of a slower rate of spread of E. maimaiga and LdNPV among larvae in these plots where the larval density was reduced 5- to 10-fold from Btk. When E. maimaiga is active, as observed in 1997, low density populations of gypsy moth may be held in check by E. maimaiga as well as by Btk applications. However, unusually dry conditions, like we observed in 1998, may suppress E. maimaiga activity and allow gypsy moth populations to build.