Farmed shrimps are vectors of various Vibrio species that are considered a potential health hazard. Previous study has shown that Vibrio parahaemolyticus, Vibrio vulnificus, and Vibrio alginolyticus dominated in the water and larval samples of shrimp hatchery (Randrianarivelo et al. 2010). The effects of two essential oils (EOs) of Cinnamosma fragrans, an endemic plant to Madagascar (B8: linalool-type and B143: 1,8-cineole-type), were determined on the total heterotrophic aerobic bacteria and the Vibrio concentrations in the rearing water of Penaeus monodon hatchery. The assays took place in OSO Farming's shrimp hatchery in Madagascar. EOs were directly added to the water tank. The bacterial concentrations of water tank were assessed on marine agar and thiosulfate citrate bile sucrose agar. The larvae culture corresponded to four replicates each of B8, B143, erythromycin (E), and control (oil and antibiotic free). The bacterial concentration of the rearing water in B8, B143, and antibiotic (E) tanks were significantly lower ( p<0.05) than in the control. Further, there was no significant difference ( p>0.05) between the three treatments B8, B143, and E. This study demonstrated that both EOs of C. fragrans, like antibiotic, inhibited bacterial growth in the rearing water of P. monodon larvae. The potential of C. fragrans EO to control the bacterial load in in vivo conditions of P. monodon hatchery makes it a relevant option for producers to minimize risk of Vibrio growth in the rearing water of larvae, which is the primary source of colonization of shrimp larvae.