The cytotoxic activity of MeOH extracts of the freeze-dried silkworm (Bombyx mori)-derived materials (4th instar larvae, female and mate pupae, virgin female and male adult), dried Beauveria bassiana-infected silkworm larvae, dried feces from the 4th instar larvae B. mori, and dried mulberry (Morus alba)-derived materials (leaves, fruits, root barks) in vitro was evaluated by sulforhodamine B assay, using the five human solid A 549 lung, SK-OV-2 ovarian, SK-MEL-2 melanoma, XF-498 CNS and HCT-15 colon tumor cell lines. The responses varied with both cell line and material used. The 70% hot MeOH extract of B. mori feces (BFH) revealed potent cytotoxic activity against model tumor cell lines whereas moderate activity was observed from the MeOH extract of B. mori feces. M. alba root barks, and M. alba fruits. The other test materials were ineffective. Because of its potent cytotoxic activity, the activity of each solvent fraction from the BFH was determined. Chloroform and ethyl acetate fractions showed the most potent cytotoxic activity. In conclusion, our results may be an indication of at least one of the pharmacological actions of B. mori feces. M. alba root barks, and M. alba fruits.