Some new 3-acetyl-5-(3-chloro-1-benzo[b]thiophen-2-yl)-2-substituted phenyl-2,3-dihydro-1,3,4-oxadiazoles and 2-(3-chloro-1-benzo[b]thiophen-2-yl)-5-substituted phenyl-1,3,4-oxadiazoles have been synthesized and evaluated for antimicrobial activity. Initially, 3-chloro-1-benzo[b]thiophene-2-carbonyl chloride (1) was prepared from cinnamic acid in the presence of chlorobenzene and thionyl chloride. This compound (1) was treated with hydrazine hydrate to afford 3-chloro-1-benzo[b]thiophene-2-carbohydrazine (2) which was further reacted with various aromatic aldehydes to yield hydrazones (3a-h). Further reaction of these hydrazones (3a-h) with acetic anhydride gave 3-acetyl-5-(3-chloro-1-benzo[b]thiophen-2-yl)-2-substituted phenyl-2,3-dihydro-1,3,4-oxadiazoles (4a-h). Reaction of the same compounds (3a-h) in the presence of chloramine-T afforded 2-(3-chloro-1-benzo[b]thiophen-2-yl)-5-substituted phenyl-1,3,4-oxadiazoles (5a-h). The structures of newly synthesized compounds (4a-h) and (5a-h) have been confirmed by spectroscopic techniques such as IR, 1H NMR and elemental analysis. All the compounds were screened for their antibacterial activities against Staphylococcus aureus, Bacillus subtilis. Escherichia coli and Pseudomonas aeruginosa and for antifungal activity against Candida albicans and Asperigillus niger. The compounds exhibited significant antibacterial and moderate antifungal activities. Compounds 4c and 4e were found to be most potent with activities, even better than standard drug ciprofloxacin against S. aureus and B. subtilis.