Three hundred and eighty first year students of the National School of Dentistry (UNAM) (n = 380) (academic year 2012-2013), were assessed targeting determination of DMFT (decayed, missing, lost teeth) index as well as to establish a relationship of whether caries is associated to Lactobacillus and Streptococcus microorganisms. DMFT index was recorded using World Health Organization (WHO) parameters. Samples of all students were taken and colony-forming units of Streptococcus and Lactobacillus were determined. DMFTindexes mean was established at 7.25 ± 4.59. Females (n = 278) and males (n = 102) exhibited mean DMFT indexes of 7.11 ± 4.66 and 7.29 ± 4.57 respectively. Results revealed that 19 year old students exhibited lesser amounts of caries than students of other ages. Both Streptococcus and Lactobacillus were significantly correlated to each other as well as to caries incidence. Increase in the number of the aforementioned micro-organisms, especiallyStreptococcus mutans, were associated to DMFT increase.