The colored urdamycins A to F, six new angucycline antibiotics produced by Streptomyces fradiae strain Tü 2717, were detected by chemical screening. They are biologically active against Gram-positive bacteria and stem cells of murine L1210 leukemia. The urdamycins are glycosides and differ in their aglycones, which can be liberated by acidic hydrolysis besides the sugars D-olivose and L-rhodinose. The structure of the main compound, urdamycin A, follows from the spectroscopic and chemical data in connection with an X-ray analysis. The aglycone urdamycinone A is identical with aquayamycin. The structures of urdamycin B, E, F and partial structures of urdamycin C and D, will be presented in a subsequent paper. The new term "angucycline/angucyclinone" is used for an increasing group of related antibiotics.