Department of Chemistry, Maharshi Dayanand University, Robtak-124 001 Manuscript received 27 November 1991, revised 24 March 1992, accepted 1 April 1992 The isomeric triazoles (6 and 8) both fused with the 2,3-bond of a thiazole, have been prepared starting from the thiosemi­carbazide (2). The triazolo[3,4-b] [1,3,4] thiadiazine (9 ; R6=C6H5 and C6H5CI-p) arises from the condensation of the triazole (5) with haloketone (R6=COCH2Br). The antibacterial and antifungal activity of the above products have been evaluated.