Abstract: Objective:To figure out the metabolic characterastics of ammonium in Pseudomonas sp. F12 and the the effect of ammonium accumulation on enzyme production and L-cysteine synthesis. Methods :The effect of ammonium on growth, DL-ATC consumption and enzyme production were performed. Besides, the effect of ammonium on bioconversion was also investigated. Results:Ammonium concentration higher than 60mmol/L inhibited growth but did not exert negative effect on DL-ATC metabolism and enzyme production. However, both ammonium and glucose existed in medium greatly decreased enzyme production. The accumulation of ammonium in bioconversion did not exert negtive effect on L-cysteine synthesis. Conclusion:the accumulation of ammonium released from DL- ATC utilization or bioconversion did not exert negative effect in enzyme production and L-cysteine synthesis.