This prospective study was designed to investigate amoxicillin-resistant oral anaerobes, and to identify their beta-lactamase-encoding genes. Three subgingival bacterial samples were collected from 12 patients suffering from periodontitis. One to seven beta-lactamase-producing strains were obtained from each patient, mostly belonging to the Prevotella genus (Bacteroides eggerthii, 2/35 strains; Prevotella sp., 33/35 strains). PCR assays were used to detect cfxA and cepA/cblA, the genes encoding class A/group2e beta-lactamases previously described in the Bacteroides fragilis group. The present investigation confirmed the role of Prevotella species as beta-lactamase producers in periodontal pockets. Additionally, this PCR screening showed (1): the high prevalence of CfxA beta-lactamase production by aminopenicillin-resistant Prevotella (32/33: 97.0% positive strains) vs. cepA/cblA (1/33: 3.0% positive strains), and (2) the presence of cfxA in the periodontal reservoir in the absence of antimicrobial therapy during the previous 6 months.