A facile synthesis of 7-[substituted]-benzylidene-3-pyridin-4-yl-[1,2,4]-triazolo-[3,4-b]-[1,3,4]-thiadiazin-6-ones have been carried out by reacting the 3-pyridin-4-yl-[1,2,4]-triazolo-[3,4-b]-[1,3,4]-thiadiazin-6-one with different aromatic aldehydes in presence of fused sodium acetate. 3-pyridin-4-yl-[1,2,4]-triazolo-[3,4-b]-[1,3,4]-thiadiazin- 6-one was prepared by reacting the mixture of chloro acetic acid and fused sodium acetate with 4-amino-3- mercapto-5-pyridin-4-yl-4H-[1,2,4]-triazole. The latter was synthesized by reacting isoniazide with carbondisulphide and potassium hydroxide followed by the addition of hydrazine hydrate. The structures of synthesized compounds have been established on the basis of chemical transformation, elemental analysis, equivalent weight determination and IR, 1H-NMR spectral studies. The title compounds have been assayed for their biological activity against gram-positive as well as gram-negative microorgansms.