Two new series of 2-alkylthio-5-aryl-1,3,4-oxadiazole (aryl = 2-hydroxyphenyl, 5-bromofuryl and alkyl = H, CH2CH3, CH2(CH2)2CH3, CH2Ph, CH2CO2CH2CH3, CH2CO2H) were synthesized and characterized by elemental analysis, UV-visible spectrophotometer, FT-IR spectrophotometer and 1H-NMR spectrophotometer. The prepared compounds were examined against two bacterial strains, Gram negative (E. coli) and Gram positive (S. aureus), and against pathogenic fungi Aspergillus niger. MIC was determined for all compounds against two bacterial strains. LD50 values were determined for some selected compounds which have showed a good antimicrobial activity. These results showed that the selected compounds exhibited moderate toxic values and the LD50 were in the range 1.85-2.9g/kg.