Abstract: The effect of benzothiazole-rifamycin on macromolecular synthesis and ultramicroscopic structure of S.aureus 209P and E. coli 2281 was reported. In both S. aureus 209P and E. coli 2281, incorporation of ³H-uridine into RNA was strongly inhibited by benzothiazole-rifamycin. Incorporation of ³H-leucine was inhibited slightly, but ³H-thymidine incorporation was not inhibited. From these results, it was inferred that benzothiazole-rifamycin primarily inhibited RNA synthesis. A weak inhibition of bacterial protein synthesis by benzothiazole-rifamycin may be accounted for by secondary effects as a consequence of the inhibition of RNA synthesis, but bacterial DNA synthesis was not inhibitied by benzothiazolerifamycin. Cultures of S. aureus were exposed to 0.005, 0.05 and 0.5 μg/ml of benzothiazole-rifamycin, the inhibition of ³H-uridine incorporation was 6.78%, 38.14% and 92o24% in 40 minutes. Cultures of E. coli were exposed to 50,100 and 200 μg/ml of benzothiazole-rifamycin, the inhibition of ³H-uridine incorporation was 12.62%, 32.04% and 45.36%.Both S. aureus 209P and E. coli 2281 were treated with benzothiazole-Rifamycin. Marked change of ultramicroscopic structure was noted in the cytoplasm which lost compact structure and vacuole appeared, while ribosomes were lost. But the cell wall was preserved.