This study was conducted in the advanced insect laboratory in the Department of Biology / College of Education for Girls / University of Kufa for the academic year 2020-2021 to evaluate the effectiveness of the fungus Beauveria bassiana in mortality percentages of the first and fourth larval instars of the red flour beetle Tribolium castanium. The results of the current study were the increase in the mortality percentages of the first larval instar with the increase in the concentrations of the fungal aqueous suspension, and the highest mortality percentage was 90% obtained on the seventh day of the experiment after being treated with a concentration of (4 %) of the aqueous extract of the fungus, while the highest mortality percentages were 80% in the fourth larval instar, after the 7 days of the experiment and the treatment at a concentration of (4 %).