The heterotrophic bacterial flora in the seawater samples from Bengal Bay and the South China Sea was surveyed in February and March, 1977. The viable bacterial number counted by the agar plate-Nuclepore filter technique ranged from 10³/100 ml at the surface to 100/100 ml at deeper layers. Often uneven, irregular vertical distribution of the viable counts was observed, and 10²/100 ml bacteria were frequently counted in samples from 800-2, 000 m layers. Bacteria of family Vibrionaceae were predominated throughout the whole sampling stations. They were particularly abundant in the water layers from 10 to 400 m. Average proportion of the Vibrionaceae among the isolated bacteria reached 32% Other bacterial genera that were found to be dominants were Pseudomonas (15/%), Acinetobacter-Moraxella (10%), Alcaligenes (7%), Flavobacterium (6%) and Micrococcus (5%). Pseudomonas spp. ware abundant at the surface of the South China Sea, comprishing 44% of the isolates from 0 m. Gram positive bactera occurred mostly at water layers deeper than 800 m.