32 species of prawn and shrimps are recorded from the Cochin Backwater. Among these, penaeus indicus  H. Milne-Edwards,  Metapenaeus monoceros (Fabricus), M.dobsoni (Miers) and  M. affinis (H. Milne-edwards) belonging to penaeidae and Macrobrachium rosenbergli (de Man) and M. idella (Hilgendorf) belonging to Palaemonidae are commercially important  and their estuarine  phase is studied. The minimum period of stay in the backwater and the size when when they leave this ecosystem are estimated to be 5 months and 50 mm for M. dobsoni; 4 months and 40 mm for M. affinis; 10 months and 85 mm for  M.monoceros  and 6 months and 80mm for P. indicus. From size frequency distribution of the juveniles in the estuary, the growth rate of each of these species is assessed and it is found that the average growth rate is highest in M.dobsoni  (9.88 mm/month) and lowest in M. monoceors (6.72 mm/month). The fresh water prawn M.rosenbergii,  is found to utilise the estuary for about 5 months. A general appraisal of the dynamics of each of the prawns in presented in the light of the data obtained from the three different ecologist niches-estuarine areas, inshore and offshore regions of the sea. Prospects of utilization of the estuarine areas for culturing different species of prawn and associated problems are discussed.