From the culture broth of the myxobacterium, Corallococcus (Myxococcus) coralloides, three new antibiotics have been isolated: corallopyronin A, B and C. The compounds, which are chemically related to the recently discovered myxopyronins, act mainly on Gram-positive bacteria, with MIC values between 0.1 and 10 micrograms/ml, and only exceptionally or at much higher concentrations (MIC values; 100 and more micrograms/ml) on Gram-negatives. They do not inhibit eukaryotic organisms and show no toxicity for mice (sc). The corallopyronins appear to block specifically eubacterial RNA polymerase.