Benzodiazepines belong to one of the most prescribed drugs worldwide. Approximatelly 1 out of 10 adults takes benzodiazepines regularly, and that they account for 50% of all psychotropic drugs prescribed. The objective of this work was to study the effect of Bromazepan on the development of the immaure stages of 3 flies of forensic importance in southeastern Brazil: Chrysomya albiceps, C. putoria and C. megacephala (Diptera: Calliphoridae). For each species four experimental groups, with three replicates each and each replicate with 150 newly emerged larvae, were set: The Control group (C), reared in the abscence of the drug, and the other 3 groups reared in the presence of: 1mg/kg (GI), 10mg/kg (GII) and 100mg/kg (GIII). The larvae in all experimental groups were reared on an artificial diet developed for immature fliesand were weighed at 12 hours intervals until pupation. The effect of the drug on the 3 species was different, depending the presence and concentration of the drug. For C. albiceps and C. putoria, larvae of group GIII weighed significantly less than the other groups (F=17.81; P<0.0001, and F=147.67; P<0.0001, respectively). For C. megacephala, all groups were statistically different from each other (F=278.05; P<0.0001), and the heavier group was GI followed by C, GII and GIII respectively. These results showed that Bromazepan had a significant influence on the blow fly larvae development, and that this influence varied according to drug concentration and to species. If not taken into consideration, this fact may lead to errors in Post Mortem Interval (PMI) estimates.