A 27-year-old woman chewed unilaterally and had restorations and caries only on the right side of the dentition. The left side had a greater amount of soft and hard deposits and more severe periodontal disease. Plaque samples from occlusal and interproximal tooth surfaces were collected and cultured. The proportion of Streptococcus mutans of the total anaerobic count in the plaque samples from the right side was 1,000 to 10,000 times as high as from the left side. The proportion of S mutans of the total streptococcus was 6% to 20% on the right and less than 1% on the left side.