A series of 3-Substituted amino-4, 5-tetramethylene thieno [2, 3-d][1, 2, 3]-triazine-4 (3H)- ones have been synthesized and characterized by UV, IR, 1H NMR, elemental and mass spectral analysis. The title compounds were evaluated for their antimicrobial activity by agar diffusion method against four bacteria and three fungi using Ampicillin and Miconazole nitrate as standards. The compounds VIIIa, IXa, Xa and XIa showed an antimicrobial ...