The aim of the present in vivo animal study is to investigate the ability of L. reuteri to colonize the oral flora during infancy. Study design: Twenty four rats, aged 1 month, which were pre-analyzed for mutans streptococci and L. reuteri absence in their saliva, were randomly divided into 3 groups. The control group was infected with S. mutans ATCC 25175 at the 2nd month, three times a day for 14 days. S. mutans counts were determined with microbiological saliva analyzes obtained by standard methods of oral swabbing at 3rd, 4th and 5th months. The second group, Probiotic I group, was also infected with S. mutans at the 2nd month, and further infected with L. reuteri ATCC 55730 (1x10(8)),5 drops per day for 25 days, at the 3rd month. S. mutans and L. reuteri counts were determined at the 3rd, 4th and 5th months. Plates were incubated anaerobically at 37 degrees C for 48 h, after which colonies were confirmed as L. Reuteri. Results: Regarding infra-group analysis, S. mutans counts of the Control group increased steadily during the 3rd. and 4th. months, and a statistically significant (p<0.05) reduction was registered at the 5. month. S. mutans counts of the Probiotic I group increased steadily during the 3rd. and 4th. months, and again a statistically significant (p<0.05) reduction, parallel with the Control group, was registered at the 5th. month. In the Probiotic II group, S. mutans counts started at a higher level than the Control group and there was a statistically significant (p<0.05) reduction of S. mutans at the 5th. month. Conclusion: It may be concluded that, L. reuteri promised a better colonization as a first colonisation strain'.