The objective of this study was to identify the oral pathogens found in odontogenic infections, to determine their susceptibilities to amoxicillin-clavulanic acid (AMC), clindamycin (CLI), doxycycline (DOX), levofloxacin (LVX), moxifloxacin (MXF), and penicillin (PEN), and to search for associations between specific pathogens and types of infection. Swabs from patients enrolled in a randomized, double-blind phase II trial comparing MXF with CLI for the treatment of odontogenic abscesses or inflammatory infiltrates were cultured on media for aerobes and anaerobes. All bacterial isolates were identified at the species level. Overall, 205 isolates were cultured from 71 patients: 77 viridans group streptococci, 56 Prevotella spp., 19 Neisseria spp., 17 Streptococcus anginosus group isolates and hemolytic streptococci, 15 other anaerobes, and 21 other bacteria. Ninety-eight percent of pathogens were susceptible to MXF, 96% to AMC, 85% to LVX, 67% to PEN, 60% to CLI, and 50% to DOX. S. anginosus group and hemolytic streptococci were found significantly more frequently (P ‫؍‬ 0.04) in patients with abscesses (12/95) than in patients with infiltrates (5/110). In four patients with infiltrates who failed to respond to CLI therapy, three isolates of the Streptococcus mitis group and four Neisseria spp. resistant to CLI were found. In this study, S. anginosus group and hemolytic streptococci were clearly associated with odontogenic abscesses. Our analysis suggests that viridans group streptococci and Neisseria spp. play a decisive role in the etiology of odontogenic infiltrates. The high in vitro activity of MXF against odontogenic bacteria corresponds well to its clinical results in the treatment of odontogenic abscesses and infiltrates.