Microbes mediating biological and many geochemical processes are the key components in marine ecosystems. Information on spatiotemporal variations in heterotrophic bacterial (Hbac) abundance and production in three ecologically disparate regions: Arabian Sea (AS), Bay of Bengal (BoB), and equatorial Indian Ocean (EIO) is provided in this chapter. In the AS, Hbac abundances were the largest during fall intermonsoon (FIM) (0.35-1.51 x 10 sup(9) L sup(-1)), moderate during southwest monsoon (SWM) (0.20-0.45 x 10 sup(9) cells L sup(-1)), and least during northeast monsoon (NEM) (0.05–0.35 x 10 sup(9) L sup(-1)). Bacterial production (BP), ranging from 2.86 to 22.93 mg C m sup(-3) d sup(-1), was the highest during spring intermonsoon (SIM) when autotrophic production was the least. In the BoB, Hbac abundance was more during FIM (0.07-1.85 x 10 sup(9) L sup(-1) in the 0- to 120-m column) and lower during SIM (0.02-0.36 x 10 sup(9) L sup(-1)). Spatial and temporal variations were clearly seen in BP too, which was higher during SWM (0.18–6.75 mg C m sup(-3) d sup(-1)) and NEM (0.35-18.56 mg C m sup(-3) d sup(-1)), moderate during FIM (0.01-4.19 mg C m sup(-3) d sup(-1)), and least during SIM (0.001-2.49 mg C m sup(-3) d sup(-1)). Annual BP accounted for up to 56% of primary production (PP) in the BoB. Despite the lowest Hbac abundance and production, BP:PP ratios were high in the oligotrophic EIO. As evidenced from sizable Hbac carbon biomass and from the often high BP:PP ratios (?1.0), Hbac-biogeochemical and trophodynamic processes are very important in some regions/seasons of low autotrophic production