The occurrence and seasonal distribution of aerobic heterotrophic bacteria in water and sediments and their ecophysiology and biochemical characters were studied for a period of three years- 1972-73, 1974-75 and 1975-76 in Cochin Backwater. The total counts between one station and the other did not show significant difference while the counts between months did. The distribution was characterised by over dispersion. Among 920 strains selected from this tropical marine environment in different months in the three years period, 23 strains were lost during purification and retransferring procedures and classified into 8 genera. The genus Alcaligenes was found to be the dominant genus in all the seasons. Data on temperature related bacterial density indicated that there are very few, true psychrophilic bacteria in Cochin Backwater and sediments. The relationship between bacterial density and nutrients are discussed.