Diel variations of hydrobiological characters were studied during a complete tidal cycle at a station in the fishing grounds at the lower reaches of Cochin Backwater during November, 1969. The tidal oscillations and the resultant tidal surface currents were found to exert profound influence on these properties. The temperature of the surface water fluctuated depending on solar radiation, whereas, the salinity variations showed a direct relationship with the tides, recording high values during flood tide. Variations in zooplankton biomass appear to be partly influenced by the diel rhythm of plankters, and the incoming tides contribute to their increase during day time. Three broad types of distribution patterns were observed for the different groups of zooplankters depending on their relationship with the tides and diel vertical migrations. Fluctuations in copepod population in the estuary evinced a complex pattern of distribution and the ingress of neritic waters and the diel rhythm were the causative factors for the diel variations in their distribution.