This study investigated the efficiency of using biofilter for ammonia removal in outdoor shrimp tanks.The experiment was divided into two parts.The first past was an incubation and evaluation of ammonia removal by biofilter under laboratory condition.The second part was the use of nitrification biofilter in outdoor shrimp tanks.During biofilter incubation,the fibrous biofilter media "Bio-CordTM" was submerged in the incubation tank containing 15 PSU seawater.Ammonium chloride at the final concentration of 2 mg-N/L was supplied as nitrogen source for nitrifying bacteria.Samples of Bio-Cord were subsequently taken out for nitrification rate measurement.It was found that nitrification rate of Bio-Cord increased with incubation time.The ammonia removal rate increased from(47.9 ± 2.6)mg NH4-N/m/day in day 22 to(106.1 ± 10.3)mg-NH4-N/m/Day in day 67.Increase of nitrate concentration in the incubation tank indicated that ammonia removal process was nitrification.Thereafter,with the second part of this study,the acclimated Bio-Cord was installed in the outdoor shrimp tanks.The tank was 2 m in diameter(3.14 m2)and filled with 1,884 L of 10 PSU seawater to the 60 cm height.Pacific white shrimps(Litopenaeus vannamei)with average weight of 0.16 0.092 g were released into the experimental tanks at 150 shrimps/m2.The experiment consisted of 2 control tanks without biofilter and 4 treatment tanks with 7 m of pre-acclimated Bio-Cord packed in the net compartments.Shrimp culture experiment was 58 days without water exchange.The results showed that chlorophyll concentration was significantly lower in treatment tanks especially during 40 days after shrimp released.This indicated the capability of nitrification biofilter that reduce phytoplankton bloom in the tanks.Inhibition of phytoplankton was possibly due to the lower ammonia concentration in the water in which the treatment tanks containing Bio-Cord-biofilter had lower ammonia concentration (1.37 ± 1.03)mg-NH4-N/L than control tanks(2.44 ± 0.02)mg-NH4-N/L.This illustrated the possibility of using nitrification biofilter under outdoor condition.