A series of substituted thiazolo [4, 5-g]-, [5, 4-g]-, [4, 5-h]-and [5, 4-h] quinoline carboxylic acids has been prepared by the following two methods with the aim of providing new antimicrobial drugs. One of the synthetic methods has been carried out through successive steps of condensation of aminobenzothiazole with diethyl ethoxymethylenemalonate, Gould-Jacobs reaction, N-alkylation and hydrolysis. The other is thiazole ring cyclization of ortho-aminated mercaptoquinoline. These compounds prepared in this work were evaluated for antimicrobial activities in vitro. 9-Chloro-8-ethyl-5, 8-dihydro-5-oxothiazolo-[4, 5-g] quinoline-6-carboxylic acid (28b) showed the highest activity.