Interaction of 5-methyl-4-oxo-3,4-dihydrothieno[2,3-d]pyrimidine-6-carboxylic acid with benzohydrazide promoted by 1,1’-carbonyldiimidazole leads to N'-benzoyl-5-methyl-4-oxo-3,4-dihydrothieno[2,3-d]pyrimidine-6- carbohydrazide. The cyclization of the hydrazide with phosphorous oxychloride resulted in the unstable 4-chloro-5- methyl-6-(5-phenyl-1,3,4-oxadiazol-2-yl)thieno[2,3-d]pyrimidine, which formed 5-methyl-6-(5-phenyl-1,3,4- oxadiazol-2-yl)thieno[2,3-d]pyrimidine-4(3H)-thione after reaction with thiourea in dimethylformamide. All of the compounds tested showed moderate unspecific antimicrobial activity against the strains of bacteria and fungi similar to Synthomycine an higher then Streptomycin against Proteus vulgaris and Pseudomonas aeruginosa.