The symbiosis of two Streptococcus mutans strains differing in carbohydrate metabolism, and a strain of Veillonella alcalescens was studied in dental plaque in gnotobiotic rats. The lactate dehydrogenase in S. mutans FIL but not in C 67-1, is independent on fructose 1,6-diphosphate. S. mutans FIL was found to produce significantly more lactic acid in plaque than C 67-1. Both strains produced acetic acid and ethanol in addition to lactic acid. The proportion of Veillonella was higher in combination with FIL than with C 67-1 (15% and 0.05, respectively). Accordingly, lactic acid was found to be significantly reduced in plaque with FIL plus Veillonella. Further, the amount of biomass was reduced in fissures containing FIL plus Veillonella. The combination of these two effects could explain the significant caries reduction observed in the rats harbouring FIL plus Veillonella. Only trace amounts of propionic acid were detected in the plaque inhabiting Veillonella.