Housefly (Musca domestica L.) maggots are used as biomedical material. Ethanolic extracts of fly maggot (EM) were orally administered to male rats at levels of 0 (control group), 4.0, 6.0, and 8.0 mg per 100 g live weight for 40 days. Serum triglycerides, total cholesterol, and LDL-C decreased by 17.90, 17.60, and 16.37%, respectively, whereas HDL-C increased by 20.48% in the EM group compared with these parameters in a control group (p<0.05). Thymus and spleen weights dose-dependently increased by 21.42% and 21.42%, respectively, but abdominal fat decreased by 39.66% after EM administration compared with that in the control group (p<0.05). IgG, IgA, and IgM increased 35.14, 68.65, and 190.16%, respectively, in the EM groups compared to the control group (p<0.05). Bifidobacterium and Lactobacillus increased by 41.68% and 35.55%, respectively, in the EM groups compared with the control group, and Bacteroides, Clostridium, Escherichia, and Streptococcus decreased by 24.96, 46.37, 25.00, and 34.05%, respectively, in the EM groups compared with the control group (p<0.05). Compared with the control group, total organic acids, acetic acid, and propionic acid increased by 31.11, 49.34, and 24.88%, whereas butyric acid, isobutyric acid, valeric acid, and isovaleric acid decreased by 30.79, 72.64, 32.90, and 63.16% respectively, in the EM groups (p<0.05). These results suggest that EM has a bifidogenic effect on immune cell development, blood lipid levels, and abdominal fat reduction by increasing the production of organic acid and numbers of cecal microorganisms in animals.