A quinocycline antibiotic, kosinostatin, was isolated from the culture broth of Micromonospora sp. TP-A0468 along with isoquinocycline B. Structure of kosinostatin was determined to be the stereoisomer of isoquinocycline B regarding to the stereochemistry at the C-2' spiro carbon by NMR analysis. Kosinostatin isomerizes to isoquinocycline B through the inversion of the stereocenter at C-2'. Comparison of physico-chemical properties indicated that kosinostatin is presumably identical with quinocycline B isolated by CELMER et al. from Streptomyces aureofaciens.