The abundance of ammonia-oxidizing bacteria and archaea and their amo A genes from the aerobic activated sludge tanks,recycled sludge and anaerobic digesters of a full-scale wastewater treatment plant(WWTP)was determined.Polymerase chain reacction and denaturing gradient gel electrophoresis were used to generate diversity profiles,which showed that each population had a consistent profile although the abundance of individual members varied.In the aerobic tanks,the ammonia-oxidizing bacterial(AOB)population was more than 350 times more abundant than the ammonia-oxidizing archaeal(AOA)population,however in the digesters,the AOA population was more than 10 times more abundant.Measuring the activity of the amoA gene expression of the two populations using RT-PCR also showed that the AOA amoA gene was more active in the digesters than in the activated sludge tanks.Using batch reactors and ddPCR,amoA activity could be measured and it was found that when the AOB amoA activity was inhibited in the anoxic reactors,the expression of the AOA amoA gene increased fourfold.This suggests that these two populations may have a cooperative relationship for the oxidation of ammonia.