An aerobic sporulating bacillus found in chernozem soils produced a thermostable antibiotic substance, colistatin, which inhibited the growth of Staphylococcus aureus and Escherichia coli. The activity of this substance was not decreased by boiling the culture fluid for 15 min. The bacteria which produce it also form a bright yellow pigment which is not antibiotic. Colistatin differs from tyrothricin and gramicidin S in that it is not pptd. by acid, and from bacitracin in that it cannot be extracted from the culture fluid by butanol. Colistatin inhibits staphylococci more strongly than E. coli. It possesses some properties in common with streptomycin. It is concentrated and purified like the latter subst. and its behavior is similar. In distinction to streptomycin, colistatin possesses purely bacteriostatic action and is not bactericidal. It inhibits the growth of staphylococci, pneumococci, E. coli, Bacillus proteus, B. paratyphosus B, B. dysenteriae Shiga, and B. typhi adbominalis.