Aims: A nonthermal atm. plasma, designed for biomedical applications, was tested for its antimicrobial activity against biofilm cultures of a key cariogenic bacterium Streptococcus mutans. Methods and Results: The Strep. mutans biofilms were grown with and without 0.15% sucrose. A chlorhexidine digluconate rinse (0.2%) was used as a pos. antimicrobial ref. The presence of sucrose and the frequency of plasma application during growth were shown to have a significant effect on the response to treatment and antibacterial activity. Conclusions: A single plasma treatment for 1 min on biofilms cultured without sucrose caused no re-growth within the observation period. However, with either single or repeated plasma treatments of 1 min, on biofilms cultured with 0.15% sucrose, growth was only reduced. Significance and Impact of the Study: In summary, there may be a role for nonthermal plasma therapies in dental procedures. Sucrose and assocd. growth conditions may be a factor in the survival of oral biofilms after treatment.