The growth inhibiting activity of materials derived from the stems of Ephedra pachyclada against intestinal bacteria was examined. Purification of the active constituent from E. pachyclada stems was conducted by silica gel chromatography and HPLC. The structure of the active component was identified as quinaldic acid ($C_{10}H_7NO_2$, 173, quinoline-2-caboxylic acid) using various spectroscopic analyses including EI-MS, $^1H$-NMR, and $^{13}C$-NMR. The growth inhibiting activity varied according to the chemical, dose, and bacterial strain tested. Quinaldic acid showed a strong () inhibition at 1.0 mg/disc against Clostridium difficile and Clostridium perfringens, and moderately inhibited the growth of C. difficile and C. perfringens at 0.5 and 0.1 mg/disc, respectively, but had no effect on the growth of Bifidobacterium bifidum, Lactobacillus acidophilus, and Lactobacillus casei. These results indicate that quinaldic acid should be considered as a possible antimicrobial agent for the treatment of diseases associated with intestinal bacteria.