Feeding behavior of the spruce budworm, Choristoneura fumiferana (Clemens), in multiple exposures to Foray 48B formulation of Bacillus thuringiensis Berliner subsp. kurstaki (Btk) was studied in the laboratory. A significant proportion of fourth- and fifth-instar larvae that were exposed to a Btk-contaminated food completely abstained from feeding on that food, even if these larvae had not previously been exposed to Btk-contaminated food. The occurrence of this behavior was higher in fourth-instar than in fifth-instar larvae and decreased with increasing fifth-instar weight. It is suggested that only larvae that have a reduced appetite can successfully abstain from feeding on the Btk-contaminated food over a 24-h period and that active ingredients of Btk or fermentation byproducts associated with the crystal/spore suspension are involved in the process. Spruce budworm did not exhibit aversion learning to Btk, which lead us to conclude that the field efficiency of multiple Btk applications should not be reduced by the behavioral response of spruce budworm larvae.