Background: Because they are rarely associated with actual infections, Bacillus species are usually defined as contaminants. However, when isolated, they require precise laboratory identification and may influence treatment. Methods: Our aims were to examine the seasonality of Bacillus spp isolates cultured from clinical blood specimens. Blood culture results of several hospitals between December 1, 2003, and October 31, 2007, were analyzed. The data were aggregated by month of isolation and were also studied by age. Spectral and Cosinor analyses were used to examine the periodicity of the bacillus isolates. Results: Of 931,331 blood cultures analyzed, 2487 (1.7%) yielded Bacillus spp isolates. There was a 2.5-fold increase in the number of bacillus isolates during August to October, compared with the other months (P < .01). This finding was consistent over hospitals and in all age groups. Spectral and Cosinor analyses confirmed this pattern. Conclusion: Isolation of Bacillus spp from blood cultures has a seasonal pattern. This observation needs to be taken into account in surveillance systems for early detection of anthrax and in investigating nosocomial outbreaks. Elucidating the cause of this seasonality may enable future reduction in contamination rates.