Objective: The purpose of the study was to compare acid production and tolerance between Candida albicans and lactobacilli strains in vitro with regard to their contribution to lesion progression in early childhood caries. Methods: 10 C. albicans strains and 18 isolates of lactobacilli (L. rhamnosus, L. paracasei ss. paracasei, L. paracasei ss. tolerance and L. delbrueckii ss. lactis) were obtained from saliva samples of caries-active children. Washed cell suspensions of the late log growth were used for repeated titrations at a constant pH in an automated pH-stat system (stat titration workstation, Radiometer Analytical SAS). Acid production was measured at a pH of 7.0, 5.5, 5.0 and 4.0, and dry cell weight as well as CFU counts were determined. Results: There were no significant differences in acidogenity between the four Lactobacillus subspecies (Tamhane test). At pH 7.0, C. albicans produced 5-fold more acid per colony forming unit than lactobacilli. Related to dry cell weight, yeast H⁺ production was 284 nmol min^-1mg^-1, 1.38-fold higher than the corresponding mean of lactobacilli (significant difference, p<0.001, Welch test). Acid formation of lactobacilli dropped linearly with pH (compared to pH 7.0: 56% at pH 5.5, 41% at pH 5.0 and 20% at pH 4.0), whereas acidogenesis of C. albicans decreased faster but regressively to 25% at pH 5.5, 15% at pH 5.0 and 10% at pH 4.0. Therefore, below pH 7.0 lactobacilli produced significantly more acid than Candida strains (p<0.0005). Conclusion: Compared on the basis of biomass, acid production and tolerance of C. albicans and lactobacilli have been found to range on a similar level. In the light of findings about frequency and proportion of C. albicans in early childhood caries lesions (Marchant et al., Caries Res 2001), its contribution to total microbial acid formation appears to be relevant for caries progression.