Novel series of 1-substituted-2-thio-(1H)-4-(3-phenylthiocarbamido-1-yl)-6-(1-substituted- guanidino-3-yl)-1,2-dihydro-s-triazine [4a(i) to 4f(ii)] has been obtained by the isomerisation of 2-(1- substitutedguanidino-3-yl)-4-(3-phenylthiocarbamido-1-yl)-6- substitutedimino-1,3,5-thiadiazine [3a(i) to 3f(ii)] in presence of ethanolic sodium bicarbonate solution, which have been obtained by basification of their hydrochlorides [2a(i) to 2f(ii)] which are synthesized by the interaction of 1-Formamidino- (n- substitutedthioamido)-5-phenyl-2-thio-4-iminobiuret (1a-f) and N-aryl/alkylisocyanodichlorides. The latter were prepared initially by the condensation of N-aryl/alkylisothiocyanate with N-phenylformamidinoformamidino-thiocarbamide. The structure of all these compounds was established on the basis of elemental analysis, IR and PMR spectral data. All the synthesized compounds have been screened for their antimicrobial activity against both gram-positive and gram-negative human pathogens.