The ecological investigations on the cyclopoid copepods, wherein various trends in the occurrence and seasonal abundance in Cochin Backwater, an estuarine system, have been studied. The material is based on regular weekly zooplankton samples collected from two fixed stations during January, 1970 to December, 1971. The various trends in the occurrence and abundance of 28 species of planktonic cyclopoid copepods belonging to 5 genera and 4 families are dealt with. The effect of salinity and temperature on their distribution is also elucidated.