The com armyworm (Spodoptera exigua Hub. Zimm.) is one of the most important pests of corn crop in Khuzestan province of Iran. Young corn plants are more susceptible to this insect defoliator. Microbial control of S. exigua specially by Bacillus thuringiensis Ber. is one of the most effective methods and is recommended by workers. This research was conducted to evaluate B. thuringiensis pathogenicity both in laboratory and field trials, in Khuzestan province. In prime step, different serotypes were evaluated by bioassay in the laboratory conditions. The results demonstrated that serotypes 1 and 3a3b were more virulent compare to third strain. In other bioassay, it was shown that second larval instars was more susceptible and along with larval growth and development, virulence of spores and crystal bodies lowered, significantly. Also, it was shown that in a few hours, affected larvae restrained significantly to feeding from corn leaves, compared with control. Application of surfactant agents such as triton x-100 in bacterial suspension increased virulence of the pathogen on associated host. In the field studies, virulence of B. thuringiensis was the same as the laboratory studies. Based on our results, B. thuringiensis can be accounted as an effective biological control agent for S. exigua in Khuzestan province.