The methods for preparation of novel derivatives of ethyl 5-(5-methyl-4-oxo-3,4-dihydrothieno[2,3-d]pyrimidin-6- yl)-1,2,4-oxadiazole-3-carboxylate, 3-substituted 5-methyl-6-(3-phenyl-1,2,4-oxadiazol-5-yl)thieno[2,3-d]pyrimidin- 4(3H)-ones and 4-(alkylthio)-5-methyl-6-(3-phenyl-1,2,4-oxadiazol-5-yl)thieno[2,3-d]pyrimidines starting from 5- methyl-4-oxo-3,4-dihydrothieno[2,3-d]pyrimidine-6-carboxylic acid have been developed. The results of the antimicrobial activity screening, performed by agar well diffusion method, for the compounds synthesized showed that ethyl 5-(5-methyl-4-oxo-3,4-dihydrothieno[2,3-d]pyrimidin-6-yl)-1,2,4-oxadiazole-3-carboxylate and its 3 pmethylbenzyl derivative together with 5-methyl-6-(3-phenyl-1,2,4-oxadiazol-5-yl)thieno[2,3-d]pyrimidine-4(3H)- thione and its 4-S-alkyl derivatives are active against the Candida albicans fungi, while 2-[5-methyl-4-oxo-6-(3- phenyl-1,2,4-oxadiazol-5-yl)thieno[2,3-d]pyrimidin-3(4H)-yl]-N-phenylacetamide significantly inhibited the growth of Staphylococcus aureus and Bacillus subtilis bacterial strains.