Objective: This study aimed to determine the relative amounts of Streptococcus mutans (SM) and lactobacilli (LBs) and their relationship with dental caries among a Yemeni adult population. Results: A positive correlation appeared between SM and LB counts from saliva and caries tissue samples and the decayed, missing and filled teeth (DMFT) score (p < 0.05). There was a significant correlation between SM and LBs in caries tissue (p < 0.05). However, there was no significant difference between SM and LBs isolated from saliva samples (p > 0.05). The number of SM and LBs in subjects with active caries was significantly higher than that in those without active caries (p < 0.05). There was no significant difference between the daily habits and SM and LB or DMFT scores (p > 0.05), except for a significant difference between brushing frequency and DMFT score (p < 0.05).