The present study investigated the -lactamase production of 73 Prevotella intermedia, 84 Prevotella nigrescens, and 14 Prevotella pallens isolates and their in vitro susceptibilities to six antimicrobial agents. The P. intermedia and P. nigrescens isolates were recovered from oral and extraoral samples obtained from subjects in two geographic locations from 1985 to 1995. The clonality of the -lactamase-positive and -lactamase-negative isolates and the clustering of the genotypes were studied by arbitrarily primed-PCR fingerprinting. -Lactamase production was detected in 29% of P. intermedia isolates, 29% of P. nigrescens isolates, and 57% of P. pallens isolates. No difference in the frequencies of -lactamase production by P. intermedia and P. nigrescens between isolates from oral and extraoral sites, between isolates obtained at different time periods, or between P. intermedia isolates from different geographic locations was observed. However, the P. nigrescens isolates from the United States were significantly more frequently (P = 0.015) -lactamase positive than those from Finland. No association between the genotypes and -lactamase production or between the genotypes and the sources of the isolates was found. The penicillin G MICs at which 90% of the isolates were inhibited were 8 µg/ml for P. intermedia, 8 µg/ml for P. nigrescens, and 16 µg/ml for P. pallens. For the -lactamase-negative isolates, the corresponding values were 0.031, 0.031, and 0.125 µg/ml, and for the -lactamase-positive isolates, the corresponding values were 16, 8, and 32 µg/ml. All isolates were susceptible to amoxicillin-clavulanate, cefoxitin, metronidazole, azithromycin, and trovafloxacin. The MICs of amoxicillin-clavulanate and cefoxitin were relatively higher for the -lactamase-positive population than for the -lactamase-negative population.