Purpose. Studies have established that toothbrushes harbor pathogenic microorganisms. This studys purpose was to investigate how covers on three powered toothbrushes affected bacterial retention. Materials and Methods. The retention of Streptococcus mutans was evaluated on the Oral-B Cross Action Power, the Sonicare Advance 4100, and the Crest SpinBrush Pro. The brushes were first sterilized with ethylene oxide. Brush heads from each brand were then submerged in a Streptococcus mutans solution for two minutes. Ten toothbrushes from each brand were covered, while 10 from each brand were allowed to air dry and were stored at room temperature. After 12 hours, five of each covered and aerated were vortexed in 10 ml sterile saline. Dilution was made (1/10), and 40 ml was placed on Mitis salivarius agar and counted after 72 hours at 37M-BM-0C. The identical procedure was carried out with the replicate 30 toothbrushes. A negative control was included. Results. Findings from the Mann-Whitney test showed that the covered brushes at 12 hours had a statistically higher number of colony forming units (CFU) than at 24 hours (p<0.0001), but uncovered toothbrushes had no significant differences at 12 and 24 hours (p=0.3620). At 12 hours, the covered brushes had significantly higher CFU than the uncovered brushes (p<0.0001), and at 24 hours no significant difference existed between the two brush types (p=0.2120). Conclusions. Toothbrush head covers affected retention of Streptococcus mutans. The longer the toothbrushes dried, bacteria levels decreased regardless of whether the brushes were covered or uncovered. Clinical Significance. All patients should be encouraged to leave toothbrushes uncovered after use to minimize bacteria survival. If patients do cover their toothbrushes, they should wait 12 hours before reuse to allow time for decreasing the bacterial load.