The temporal dynamics of Bartonella infections in a rodent community were described by repeatedly capturing and sampling individual animals. Among six rodent species, from which bartonellae were isolated, cotton rats (Sigmodon hispidus) accounted for > 98% of the bacteremic animals. All cotton rats captured four or more times were Bartonella-culture positive at least once. The lowest monthly prevalence of Bartonella in cotton rats was in June (49%) and the highest was in October (95%). Prevalence of Bartonella infection increased to > 90% among juvenile and subadult rats before declining to < 40% among the largest-oldest individuals. Bacteremia levels ranged between 40 and 4.0 x 10(6) colony forming units (CFU) per 1 mL of blood. Male cotton rats had significantly higher CFUs than females (p = 0.006). The median of Bartonella bacteremia decreased monotonically by age group among cotton rats. Although Bartonella infections were highly prevalent among cotton rats, only 8.5% of rats had reactive antibodies at titers of > or = 1:32 and none had antibodies titers of > 1:256.