Objective To explore the bacterial species and total number of bacteria in seawater around an island in Xi-Sha archipelago and evaluate the drug sensitivity of the bacteria. Methods We studied the bacteria in various sea districts of the island. We collected 80 samples of seawater and cultured to identify bacterial species and their drug sensitivity. Results Vibrio ranked first on the isolation rate (45.98%), and from the near to distant sea districts, its isolation rate presented a typical stepwise upward tendency. The isolation rates of Sphingomonas paucimobilis and Moraxella species presented a typical stepwise downward tendency in contrast to Vibrio alginolyticus. CDC group EO-2 was mainly found in near sea districts but not along the seashore or in distant sea districts. The average bacteria number was 337.44?174.88 cfu/ml. The quantities of bacteria presented an obvious gradient downward trend from the near to distant sea districts. Vibrio cholerae, Samenella. dysenteridae, E.coli and Staphylococcus aureus were not detected. The four main kinds of bacteria found by us were all highly susceptible to 16 kinds of antibiotics, and we found no resistant strains to Tetracycline, Gentanycin. Chloromycin, and Amikacin. Conclusions The quality of seawater in Xi-Sha was fine in so far as bacterial contamination was concerned. Main bacterial species found included Vibrio alginolyticus, Sphingomonas paucimobilis, Moraxella species and Comamonas terrigena etc. The isolation rate of the different bacterial species presented obvious gradient variations. In sea districts far off the seashore, Vibrio alginolyticus accounted for 77.27% of the all bacteria. All the main bacterial species were highly sensitive to antibiotics.