Kosinostatin, a quinocycline antibiotic was isolated from the culture broth of an actinomycete strain TP-A0468 along with isoquinocycline B. The producing strain was isolated from the seawater sample collected in Toyama Bay and identified as Micromonospora sp. based on the taxonomic study. Kosinostatin was obtained from the culture fluid by solvent extraction and ODS column chromatography. Kosinostatin inhibited the growth of Gram-positive bacteria strongly (MIC=0.039 microg/ml) and Gram-negative bacteria and yeasts moderately (MIC= 1.56 approximately 12.5 microg/ml). It showed cytotoxicity against various cancer cell lines with the IC50 of 0.02 approximately 0.6 microm and inhibited human DNA topoisomerase Ila with the IC50 of 3 approximately 10 microM.