Seasonal diel variations of nitrogen-fixation in fish ponds under different manurial management practices were studied covering four seasons of a year. Water quality and bacterial populations with reference to aerobic heterotrophs and aerobic nitrogen-fixers were also assessed along with nitrogen-fixation rates at quarterly intervals covering all the seasons from 0900 hrs to 0900 hrs of the next day. The manurial treatments comprised pond fertilization with cowdung at 10 t/ha, poultry manure at 2 t/ha and inorganic fertilizers at 100 kg N and 50 kg P /ha/yr, along with a control and a natural swamp without any application. The nitrogen-fixation rates in terms of ARA increased during day and decreased during night hours. In general, poultry manure treatment showed higher nitrogen fixation activity than other fertilization measures.Nitrogen-fixation rates in terms of ARA in the water medium of the ponds during diel studies were in the ranges of 3.20-15.67, 3.09-16.47, 4.20-20.11, 3.22-20.17 and 0-16.26 n mol C2H4/l/hr. The nitrogen-fixation rates increased from morning till afternoon (3.72-20.11 n mol C2H4/l/hr) and then decreased to 1.83 n mol C2H4 /l/hr except during February, where marginal variations were observed, with the rates increasing towards night hours. The natural swamp, showed a similar trend with increased values till afternoon (3.97-6.37 n mol C2H4/l/hr) and subsequently reduced towards dawn (3.67-3.81 n mol C2H4 /l/hr). The diel variations of ARA in different treatments were significant between hours in February (F6,24=7.87;p<0.001), May (F6,24= 9.04; p<0.001), August (F6,24=2.17;p<0.10) and November (F6,24= 2.80;p<0.05).