Abstract: A study was conducted to estimate the bacterial load as well as diversity in the larvae and rearing system of the marine blue swimmer crab Portunus pelagicus. Berried females of mean carapace width 159.86±3.04 mm were collected from the wild and the zoeae from a single brood were used for the investigation. The bacterial load in the filtered seawater, rearing seawater, live and dead larval tissue were estimated and the bacterial species diversity was identified following standard identification keys based on the morphological, phenotypic as well as biochemical characteristics of the colonies isolated on Trypticase Soy Agar (TSA) and Thiosulphate Citrate Bilesalt Sucrose agar (TCBS). The bacterial loads recorded were 1.2x104, 2.9x105 CFU mL-1, 2.3x105 and 4.8x107 CFU g-1 for filtered seawater, rearing seawater, live larvae and dead larvae, respectively. The dominant bacterial strains identified were Vibrio vulnificus and Vibrio alginolyticus in rearing seawater; Pseudomonas alcaligenes, Pseudomonas acidovorans, Bacillus marinus and Alcaligenes sp. in live larval tissue homogenate and Vibrio splendidus, V. vulnificus, V. alginolyticus and Vibrio parahaemolyticus in dead larval tissue homogenate. No growth was observed on TSA and TCBS plates inoculated with filtered seawater.