OBJECTIVE: Acetaldehyde, associated with consumption of alcoholic beverages, is known to be a carcinogen and to be related to the tongue dorsum. The aim of this study was to investigate the relationship between acetaldehyde concentration in mouth air and bacterial characteristics on the tongue dorsum. METHODOLOGY: Thirty-nine healthy volunteers participated in the study. Acetaldehyde concentrations in mouth air were evaluated by a high-sensitivity semiconductor gas sensor. A 16S rRNA gene sequencing technique was used to compare microbiomes between two groups, focusing on the six samples with the highest acetaldehyde concentrations (HG) and the six samples with lowest acetaldehyde concentrations (LG). RESULTS: Acetaldehyde concentration increased in correlation with the increase in bacterial count (p=0.048). The number of species observed in the oral microbiome of the HG was higher than that in the oral microbiome of the LG (p=0.011). The relative abundances of Gemella sanguinis, Veillonella parvula and Neisseria flavescens in the oral microbiome of the HG were higher than those in the oral microbiome of the LG (p<0.05). CONCLUSION: Acetaldehyde concentration in mouth air was associated with bacterial count, diversity of microbiome, and relative abundance of G. sanguinis, V. parvula, and N. flavescens.