The effects ofBacillus thuringiensisBerliner in artificial diet on the feeding behavior of lalvae of the gypsy moth,Lymontria dispar(L.), were measured in both short-term (1 d) and long-term (10 d) tests. In short-term tests, reduced rates of feeding, growth, and digestive efficiency were caused byB. thuringiensisat concentrations that did not kill larvae within the duration of the tests. In long-term tests, larvae given a choice between treated and untreated diets avoided diet containingB. thuringiensisfor the first 7 d. After 7 d, increased movement by the larvae apparently obscured effects ofB. thuringiensison the distribution of the larvae. Effects ofB. thuringiensison the feeding behavior of gypsy moth larvae may, therefore, be one reason why high rates of mortality of gypsy moth are difficult to obtain withB. thuringiensisunder field conditions.