A series of novel ([1,2,4]triazolo[1,5-c]quinazolin-2-ylthio)carboxylic acids 2a-d and esters 3a-l were synthesized and evaluated for antimicrobial activity. Alkylation of potassium 2-thio-[1,2,4]triazolo[1,5-c]quinazoline 1 with halogenocarboxylic acids and its esters proceeded S-regioselectively. During acid catalyzed esterification of 2a-c, degradation of the pyrimidine ring was observed. The structures of the compounds were elucidated by FT-IR, (1)H- and (13)C-NMR, electron impact mass spectra (EI-MS) and LC-MS spectral data. Antimicrobial and antifungal activity of synthesized compounds was tested against Escherichia coli, Pseudomonas aeruginosa, Aspergillus niger, Mycobacterium luteum, Candida albicans and Candida tenuis. Acids 2a and 2c exhibited significant activity against C. albicans, which was additionally confirmed by the bioluminescence inhibition test and interrelated with their lipophilicity.