Cycloalkano-quinolone derivatives having anti-microbial activity characterized by the formula: ##SPC1##Wherein one of R.sub.1 and R.sub.2 designates hydrogen, halogen, nitro, amino, hydroxyl, acylamino, acyloxy, alkoxy or aryloxy and the other taken together with R.sub.3 forms an akylene bridge containing 2 to 6 carbon atoms, X is hydrogen, lower alkyl, which may be substituted by a member of the group consisting of halogen, hydroxyl, acyloxy, alkoxy, aryloxy, aralkyloxy, mercapto, alkylmercapto, arylmercapto, arylsulfonyl, and alkylsulfonyl, or X is unsubstituted alkenyl including alkadienyl, or alkynyl and Y is hydroxyl, amino or lower alkoxy, and the pharmaceutically acceptable salts thereof.