Triazole and triazoles fused with six-membered ring systems are found to possess diverse applications in the fields of medicine, agriculture and industry. The new 1,2,4-triazole and 1,2,4-triazolo[3,4-b][1,3,4]thiadiazine derivatives were synthesized as novel antimicrobial agents. The reaction of 1H-indol-3-acetic acid with thiocarbohydrazide gave the 4-amino-3-mercapto-5-[(1H-indol-3-yl)methyl]-4H-1,2,4-triazole. The reaction of triazole with arylaldehydes in ethanol gave the 4-arylideneamino-3-mercapto-5-[(1H-indol-3-yl)methyl]-4H-1,2,4-triazoies (1). The 3[(1H-indol-3-yl)methyl]-6-aryl-7H-1,2,4-triazolo[3,4-b]]1,3,4]thiadiazines II) were obtained by condensing triazole with phenacyl bromides in absolute ethanol. The chemical structures of the compounds were elucidated by IR, H-1 NMR and FAB(+)-MS spectral data. Their antimicrobial activities against Micrococcus luteus (NRLL B-4375), Bacillus cereus (NRRL B-3711), Proteus vulgaris (NRRL B-123), Salmonella typhimurium (NRRL B-4420), Staphylococcus aureus (NRRL B-767), Escherichia coli (NRRL B-3704), Candida albicans and Candida glabrata (isolates obtained from Osmangazi University, Faculty of Medicine) were investigated and significant activity was obtained. (C) 2007 Elsevier Masson SAS. All rights reserved.