Objective:The main objective is to investigate the antibacterial effect of diode laser against Aggregatibacter actinomycetemcomitans biofilm.Materials and Methods:Biofilms of A. actinomycetemcomitans plus Streptococcus sanguinis grown on bovine root surfaces were treated with an 810-nm diode laser using a noncontact pulsed mode with a pulse interval and pulse length of 20 ms. Four protocols, that is, one episode of 1.5 or 2.5 W for 30 s and three episodes of 1.5 or 2.5 W for 30 s were tested. No treatment and 0.2% chlorhexidine treatment served as negative and positive controls, respectively. Viable bacterial number was determined by colony counting.Results:Treatment with chlorhexidine and all laser protocols except that using single episode of 1.5 W reduced the number of A. actinomycetemcomitans in either single-species or dual-species biofilm compared to negative control. A higher percentage of A. actinomycetemcomitans reduction was demonstrated after increasing the power output or repeating the irradiation.Conclusions:The laser protocols used in this study could reduce the number of viable bacteria but not eradicate A. actinomycetemcomitans biofilm.