Arginine is an important pH-elevating agent in the oral cavity. It has been incorporated in oral hygiene formulations to mitigate sensitivity and to prevent caries. In this investigation, the effects of sustained arginine dosing of dental plaque microcosms on bacteriological composition and pH were evaluated under controlled conditions. Plaque microcosms were established in constant depth film fermenters using salivary inocula and fed continuously with artificial saliva. To simulate resting and cariogenic states the CDFFs were supplemented with sterile water or 5% sucrose, respectively. Plaques were then dosed twice daily with a dentifrice with (DA) and without (DN) added arginine (at 1.5%). This regime continued for over three weeks, after which fermenters were maintained without dosing. Microcosms were analyzed by differential viable counting, with a pH microelectrode and by eubacterial DNA profiling. Sucrose dosing was associated with significantly decreased pH (p=<0.001), significantly (p=<0.05) increased counts of total aerobes, Gram negative anaerobes, aciduric species, acidogenic species, arginine utilizing species, bifidobacteria, lactobacilli and streptococci and significant (p=<0.05) changes in DNA profiles. Plaques dosed with DA had a significantly (p=<0.001) higher pH than those dosed with DN, with or without sucrose supplementation. Dosing with DA but not DN significantly decreased (p=<0.05) counts of all functional bacterial groups apart from the total anaerobes in cariogenic plaques, and in resting plaques, significantly (p=<0.05) decreased counts of streptococci, lactobacilli, bifidobacteria and acidogenic bacteria. In summary, sustained exposure of oral microcosms to arginine in formulation significantly increased plaque pH and significantly reduced the viability of cariogenic bacterial species.