The present study deals with the diel variations of larval forms in the surface and bottom waters at two stations in the Coleroon Estuarine Complex situated at Lat. 11°21' N ; Long. 79° 09' E. The data were obtained for 36 hrs of sampling made at 6 hours intervals. Both surface and bottom water samples were analysed for dissolved oxygen, salinity, pH, alkalinity and nutrients.

The number of larvae ranged between 956 organisms/m» (1745 hrs at low tide) and 2592 organisms/m³ (2345 hrs at high tide) in the surface water and between 914 organisms/m³ (1745 hrs at low tide) and 3542 organisms/m³ (2345 hrs at high tide) in the bottom at Station I. At Station n it ranged from 1052 organisms/m³ (1815 hrs at low tide) to 3492 organisms/m³ (0015 hrs at high tide) in'the surface water and from 977 organisms/m³ (1800 hrs at low tide) to 5058 organisms /m³ (0015 hrs at high tide) in the bottom water.

The larval forms collected belonged to prawn, polychaete, fish, crab, copepod, cirripede, lamellibranchs and gastropods. The percentage composition of prawn larvae was found to be high both in surface (418/during high tide, 1215 hrs at Station II) and bottom (67.2/during low tide, 0615 hrs at Station II) waters. Further, the density of the larvae was high at midnight (2345 hrs) and low during evening hours (1745 and 1815 hrs). Between stations, Station II was found to have a large number of larval forms than Station I. Dissolved nutrients were also found to be more at Station II when compared to Station I. It was inferred that larval recruitment in Coleroon Estuary occurs mainly due to the influx of Pitchavaram mangrove waters at low tide and coastal water at hightide, during night.

The variations in the environmental parameters in relation to the densitites of larval forms at the two stations were analysed statistically and discussed.