Several 3-aryl/alkylimino-6-phenylimino-3,4,5,6-tetrahydro-1,2,4,5-dithiadiazines (IV) have been synthesized by the interaction of 4-aryl/alkyl-3-thiosemicarbazide (I) and N-phenyl S-chloro isothiocarbarnoyl chloride (II),Compounds (IV) on acetylation in 1:1 ratio gave monoacetyl derivatives (V). The title compounds were tested for their antimicrobial activity against gram positive and gram negative microorganisms,