The carcinogenic effect of bracken fern (Pteris aquilina) to the urinary bladder of mice was studied. Powdered bracken fern mixed with a basic commercial diet (1:4 by weight) was fed to a group of 40 ICR strain mice for 20 weeks after the implantation of a glass bead into the bladder. A total experimental period was 70 weeks. Fifteen mice survived 40 or more weeks and 4 mice (27%) developed urinary bladder tumors. No urinary bladder tumor was found in the group of mice fed a similar bracken fern diet mixture without a glass bead in their bladder and in the control group of mice fed a normal diet after the implantation of a glass bead. The result of this experiment may indicate that there is a metabolite(s) in the urine of bracken-fed mice carcinogenic to the bladder.