Disease in spruce budworm,Choristoneura fumiferana(Clemens), caused by the microsporidianNosema fumiferanae(Thomson), increased larval susceptibility to mortality byBacillus thuringiensis(Berliner) treatments compared with larvae free ofN. fumiferanaedisease. The median lethal concentration (LC50) ofB. thuringiensis, as determined by the diet incorporation bioassay method, was significantly lower for larvae infected transovarially withN. fumiferanae, but the similar slope obtained for initially healthy larvae indicated an independent and additive interaction. The median lethal time (LT50) forB. thuringiensiswas shortest for the group diseased withN. fumiferanae. Results from separate and sequential oral treatments (horizontal transmission) of both microorganisms at fixed physiological stages support the findings from the transovarial studies. It alsowas determined thatN. fumiferanae-free larvae survivingB. thuringiensistreatments were more susceptible to mortality from subsequent inoculations withN. fumiferanaethan were larvae not previously exposed toB. thuringiensis.