Abstract Introduction and Objective: Antibiotics have been used as an adjunct to periodontal treatment. Amoxicillin and amoxicillin/clavulanic acid are some of the most used. To define the type of bacteria and their susceptibility to amoxycillin and to amoxicillin/clavulanic acid in patients with aggressive periodontitis. Materials and Method: This descriptive study stated susceptibility to amoxycillin and to amoxicillin/clavulanic acid of bacteria isolated from 19 patients with aggressive periodontitis, selected by convenience. Samples from six (6) different sites with periodontal pockets equal or greater than 7 mm of depth were obtained. The samples were cultured anaerobically and the isolated colonies were identified by means of the Gram staining procedure and the Crystal System for anaerobics. The Wilkins and Thiel test was used to assess susceptibility. Results: The microorganisms with the highest presence in localized periodontitis were Veillonella species, Peptostreptococcus prevotii, Staphylococcus saccaharolyticus, Bacteroides fragilis and Bacteroides ureolyticus. Fusobacterium nucleatum, Veillonella species, Peptostreptococcus prevotii, Peptostreptococcus saccharolyticus and Staphylococcus saccharolyticus were found in generalized periodontitis. Four out of the 21 microorganisms species identified in localized periodontitis 4 were resistant to amoxycillin and one to amoxicillin/clavulanic acid. One microorganism was identified as resistant to amoxycillin and one to amoxicillin/clavulanic acid in generalized periodontitis. Conclusion: Amoxycillin-resistant and amoxycillin/clavulanic acid-resistant bacteria were found.