Thiazolo[5,4-f]quinoline-8-carboxylic acid derivatives having either the general formula (I): ##SPC1## wherein R.sub.1 is a lower alkyl group, a lower alkenyl group or a benzyl group; or the general formula (III-a): ##SPC2## Wherein R.sub.3 is a lower alkyl group and X is a halogen atom; and methods for the preparation thereof. The compounds of the general formula (I) are useful as antibacterial agents against gram-negative and gram-positive bacteria. The compounds of the general formula (III-a) are useful as antifungal agents against eumycetes and also as an intermediate in a preparation of the compounds of the general formula (I).