Major changes in the hydrobiological conditions of Cochin Backwater are controlled by the tidal movements and also by the amount of freshwater discharged into the system during the S. W. Monsoon. Whereas the salinity changes are affected mostly by the tidal incursions, those of temperature and oxygen are influenced by tide and time of the day. The oxygen values are found to remain undersaturated during monsoon period. This may be due to the utilisation of oxygen for the decomposition of dead planktonic organisms brought down by flood tide or silt loaded fresh water. The decomposition is greater when the fluctuations in salinity are wider. The inorganic phosphate has no direct relationship with the time or tide but the nitrite values appear to follow high salinity values particularly along the bottom.