Determination of the steps in mode of action of Bacillus thuringiensis var. tenebrionis on the midgut of Leptinotarsa decemlineata (Say.) (Coleoptera: Chrysomelidae) larvae The steps in mode of action of δ-endotoxin with insecticidal efficacy produced by gram-positive bacteria Bacillus thuringiensis var. tenebrionis were described in details. Influences of the bacteria applied on last instars larvae of colorado potato beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae) were observed on brush border membrane of midgut. A group of the fresh last instars larvae were fed with treated leaves by Novodor FC, commercial bacteria formulation, with recommended doses while other groups were being fed with treated leaves by distillated water. Fixations were done with two hours intervals at the first twelve hours after treatment. At following twelve hours fixations periods increased to six hours intervals and twelve hours intervals at following another twenty four hours. Effects of bacteria were evaluated from slides prepared by histological methods from the fixated samples. According to the results, first symptoms of effects of bacterial toxin on microvillus were observed after two hours from the beginning of feeding. Effects were becoming widespread and stronger at the end of four and six hours. In the samples taken following hours, effects were increased gradually and especially significant damages were detected on brush border membrane and midgut cells after twelve hours