Oreochromis niloticus fingerlings were stocked at densities of 50, 100, 150 and 200 fish/m super(2) in fixed net cages in Laguna Lake, Philippines. These were reared without supplemental feed on 1 April-30 July 1980, 5 August-3 December 1980, and 19 December 1980-19 April 1981. Water temperature, dissolved oxygen, chloride level and pH for the three periods were measured outside the net cages. Growth rates were highest at 50 and 100 fish/m super(2) reared between April and July when phytoplankton biomass was high with a peak production observed in June (65.5 g/m super(2)), and zooplankton biomass ranged from 9.3 to 32.68 g/m super(2). Under these conditions, fish initially weighing 1.6-1.7 g grew to a marketable size of 106-124 g in four months. Those stocked at 150 and 200 fish/m super(2) had lower weights but still weighed over 80 g. None of those in the other two rearing periods exceeded 50 g after four months. Survival and fish yield during April-July were also higher than for those reared during the other periods.