The effect of orthodontic treatment on the prevalence of Streptococcus mutans was investigated in 14 subjects. Insertion of appliances tended to give a transient decrease in S.mutans levels, in both plaque and saliva, possibly due to elimination of S.mutans reservoirs by the banding procedure. After 3 months of extensive banding, the S.mutans proportions surpassed pretreatment levels in saliva and on banded teeth, whereas unbanded surfaces only showed a slight increase. It was thus concluded that creation of new retentive areas favors the local growth of S.mutans, which in turn increases the general infection level of this organism. Time was, however, needed before elevated levels were reached. Insertion of appliances tended to eliminate the characteristic gradient of S.mutans levels within the dental arches.