The growth responses of nine human intestinal bacteria to liquid culture of Cordyceps militaris Link. Pt. (Ascomycotina: Clavicipitaceae) collected from a pupa of Bombyx mori L. (Lepidoptera: Bombycidae) were examined using spectrophotometric and impregnated paper disk methods and compared to those of tetracycline and chloramphenicol, as well as those of Coptis japonica root-derived berberine chloride. The biologically active constituent of the cultures was characterized as cordycepin (3'-deoxyadenosine) by spectroscopic analysis. This compound revealed potent growth-inhibiting activity toward Clostridium paraputrificum and Clostridium perfringens at 10 microgram/disk without adverse effects on the growth of Bifidobacterium bifidum, Bifidobacterium breve, Bifidobacterium longum, Bifidobacterium adolescentis, Lactobacillus acidophilus, and Lactobacillus casei, whereas tetracycline and chloramphenicol inhibited the growth of these lactic acid-producing bacteria, clostridia and Escherichia coli. However, C. militaris-derived materials revealed no growth stimulation on the bifidobacteria and lactobacilli. These results may be an indication of at least one of the pharmacological actions of C. militaris. As a naturally occurring antibacterial agent, cordycepin could be useful as a new preventive agent against various diseases caused by clostridia.