Department of Chemistry, Maharshi Dayanand University, Rohtak-124 001 Manuscript received 7 November 1988, accepted 19 January 1989 6-Chloro-5H -2,3-dihydro[1,2 4]triazino[5,6 b]indole-3-thione (2) on condensation with \(\propto\) -haloketones gives 5H-3-aroylmethylthio-6-chloror[1,2,4]triazino[5,6-b]indole hydrohalides (3) which on PPA-catatysed cyclisation furnish 3-aryl-9-chlorothiazolo[ 3',2' :2,3][1 2,4] triazino[5,6-b]indoles (4) and not the augular isomeric 1-aryl-9- chlorothiazolo [2',3' : 3,4] [1,2,4]triazino[5,6·b]indoles (6). The unequivocal synthesis of 6 has also been accomplished. The antibacterial and antifungal activities of some or the compounds have been evaluated.