The environmental isolate Kytococcus sedentarius TR-2 was found to be a new producer of the oligoketide antibiotics monensin A and B. Electron microscopic studies demonstrated that the TR-2 strain had coccoid cells and DNA analysis revealed no close relationship to Streptomyces cinnamonensis, a typical monensin producer. Production of monensins was also proven with six culture collection K. sedentarius strains and three Dermacoccus nishinomiyaensis strains. The secondary metabolism of micrococci demonstrates a high degree of instability. Biosynthesis of monensins by micrococci endorses a phylogenetic relationship to Streptomyces spp.