Human teeth in an artificial mouth were inoculated withStreptococcus mutansBHT,Streptococcus mitiorLPA‐1, or sequentially with both organisms. Incubation was continued for 90 h. Mixed populations were largest when a nutrient supplement containing 5–0% (w/v) sucrose was supplied. Fewer organisms were recovered from experiments with synthetic saliva only, or when a supplement containing 0–05% (w/v) glucose was available. The inoculation sequence determined the total viable count and a larger population resulted whenStrep, mutanswas the initial colonizer (P<0–01).Strep, mutanswas always able to become established even when super‐infected on to a 24 h plaque ofStrep, mitior.The final proportion ofStrep mutanswas lower when it was the superinfecting organism and the sucrose (P<0–01) or glucose (P<0–05) nutrient supplement was provided. This work confirms the importance of inoculation sequence and presence of sugars in plaque accumulation and demonstrates the fundamental role of microbial interactions in