Microflora in tank waters of prawn culture at salinity 10, 20, 30, 40, and 50ppt were investigated. Two groups of grass prawn: larvae and middle size of prawn were kept in a tank at different salinity. Water samples were collected before introduction of the prawn, and after introduction in 2, 20, and 50 days. Before the larvae were transferred, Moraxella and Pseudomonas predominated in salinity 10 and 20ppt, Vibrio in 30ppt, Pseudomonas in 40ppt, and Pseudomonas and Moraxella in 50ppt. After larvae introduced in two days later, Vibrio and Pseudomonas predominated in salinity 10, 20, and 30ppt; but Pseudomoas and Vibrio in 40 and 50ppt. The predominant genera became Moraxella in 10ppt, Vibrio in 20 and 30ppt, Acinetobacter in 40ppt, and Pseudomonas in 50ppt after 20 days. However, the variation changed after 50 days of larvae introduction. Vibrio and Pseudomonas became dominant in 10ppt. The predominant genera were Aeromonas, Flavobacterium, Vibrio, and Flavobacterium in salinity 20, 30, 40, and 50ppt, respectively.The microflora found in tanks holding middle size of grass prawn was relatively stable at the beginning. The predominant genus was Flavobacterium in salinity 10, 20 and 30ppt for the first 20 days when the prawns were transferred. However, Flavobacterium was replaced by Vibrio in 40 and 50ppt in 20 days. After 50 days, the microflora were different. The predominant genera were Aeromonas in 10 and 20ppt, Alcaligenes in 30ppt, Vibrio in 40ppt, and Vibrio and Pseudomonas in 50ppt.