In order to search for new antimicrobial agents, a number of 2-substituted 8-ethyl-5, 8-dihydro-5-oxothiazolo [5, 4-b] naphthyridine-6-carboxylic acid and their related compounds which consist of a new ring system were prepared. Reactions of the 6-amino-1-ethyl-7-mercapto-1, 4-dihydro-4-oxonaphthyridine-3-carboxylic acid (2) with acid, acid anhydride, acid chloride and ethylxanthate afforded several thiazolo [5, 4-b] naphthyridine derivatives. Refluxing ethyl 7-chloro-1-ethyl-6-nitro-1, 4-dihydro-4-oxonaphthyridine-3-carboxylate (16) with KSCN in acetic acid gave directly the thiazole cyclization product 19. Reaction of 8-ethyl-2-methylthio-5, 8-dihydro-5-oxothiazolo [5, 4-b] naphthyridine-6-carboxylic acid (11) with dimethyl sulfate gave various products (13b, 25 and 26) depending upon its reaction conditions. Some compounds obtained in this work exhibited high activities against gram-negative and gram-positive bacteria in vitro.