Aims: To determine the influence of oxygen exposure and growth stages on oxygen tolerance for clinical and reference specimens of the genus Prevotella. Methods and Results: Tolerance to oxygen exposure was evaluated along growth stages for a total of four Prevotella isolates constituted by two strains tested soon after isolation from periodontally compromised patients and two reference strains kept frozen (-86degreesC) in our laboratory collection. Additionally, the recently recovered isolates were also assayed after 4 months exposed to oxygen during laboratory handling. On solid medium, recently recovered Prevotella specimens proved to be less tolerant to oxygen exposure soon after isolation than when exposed for 4 months to oxygen during laboratory handling. Oxygen resistance of reference strains maintained in the laboratory collection showed to be the highest when compared with the clinical isolates both before and after oxygen exposure. When oxygen tolerance was tested during growth, bacterial cells from the exponential phase were more tolerant. Differences were observed in protein patterns between clinical isolates soon after recovery and after laboratory handling as determined by SDS-PAGE of crude cell-free extracts. However, SOD activities were similar in all bacterial samples tested. Conclusions: The ability to adaptively change oxygen tolerance was observed for pathogenic specimens of Prevotella . This property may be considered a virulence factor. Significance and Impact of the Study: Adaptative aerotolerance of Prevotella as a factor for virulence and survival.