As a part of systematic investigation of synthesis and biological activity of several new 2-[2- (Substituedbenzyliden)hydrazinoacetyl]-mercapto-5-methyl-1,3,4-thiadiazole, 3(a-n), 2-[2-{2- (substitutedphenyl)-4-oxo-1,3-thiazolidin}–acetylimino]-mercapto-5-methyl-1,3,4-thiadizole, 4(a-n) and 2-[2- {2-(Substitutedphenyl)-4-oxo-5-(substitutedaryliden)-1,3-thiazolidin}-acetylimino]-mercapto-5-methyl-1,3,4- thiadiazole, 5(a-n) have been synthesized from 2-(2-hydrazinoacetyl)-mercapto-5-methyl-1,3,4- thiadiazole, 2 using 2-mercapto-5-methyl-1,3,4-thiadiazole as the starting material. All the synthesized products were evaluated for their and antibacterial activity against B. substilis, E. coli. K. pneumoniae and S. aureus bacteria and antifungal activity against A. niger, A. flavus, F. oxisporium and T. viride fungi respectively. The structures of all the synthesized compounds have been determined by their spectral and microanalytical data.