Thein vitroantibacterial activity of essential oils (EOs) obtained from fresh bulbs of garlic,Allium sativumL., and leek,Allium porrumL. ( Alliaceae), was studied.A. sativum(garlic) EO showed a good antimicrobial activity againstStaphylococcus aureus(inhibition zone 14.8 mm),Pseudomonas aeruginosa(inhibition zone 21.1 mm), andEscherichia coli(inhibition zone 11.0 mm), whereas the EO ofA. porrum(leek) had no antimicrobial activity. The main constituents of the garlic EO were diallyl monosulfide, diallyl disulfide (DADS), diallyl trisulfide, and diallyl tetrasulfide. The EO ofA. porrumwas characterized by the presence of dipropyl disulfide (DPDS), dipropyl trisulfide, and dipropyl tetrasulfide. The antimicrobial activities of the DADS and DPDS were also studied. The results obtained suggest that the presence of the allyl group is fundamental for the antimicrobial activity of these sulfide derivatives when they are present inAlliumor in other species (DADS inhibition zone onS. aureus15.9 mm,P. aeruginosa21.9 mm,E. coli11.4 mm).