In the course of searching for antibiotics we have isolated from a strain of Pseudomonas fluorescens a novel compound that possesses potent broad-spectrum antibacterial activity. We now wish to report the results of our studies on the taxonomy, isolation, biological and physical properties of this compound, EM5519, and the results of preliminary studies on its mode of action, which indicate that the compound inhibits primarily RNA synthesis. From the data given, we believe that EM5519 is a new compound from a pseudomonad with potent antibiotic activity. When our studies were being brought to conclusion, we became aware of a patent granted to MUNAKATA et al. of Yoshitomi Pharmaceutical Industries, Ltd., concerning their novel antibiotics, Y-16482 a and β. TLC and 1H NMR spectroscopic comparisons of EM5519 with these compounds, kindly supplied by Yoshitomi, proved the identity of EM5519 with Y-16482 a.