||The use of seawater for domestic purposes is progressively increasing in order to minimize the wastage of fresh water from unnecessary usage such as lavatory flushing especially for those locations without adequate fresh water supply. However, the salinity of wastewater also increases accordingly, which will adversely affect the slowly growing and environmentally sensitive nitrifying bacteria in biological wastewater treatment. Therefore, development of saline-resistant nitrifying sludge is necessary. In order to realize this goal, responses of nitrification processes as well as the nitrifying population to the change of salinity in wastewater should be evaluated. With the above in mind, this study is aimed at examining the effect of increasing salinity on both nitrification and nitrifiers. The effect of chloride on the responses of nitrifying activity, ammonia removal efficiency, and nitrifier population to increasing chloride level were investigated using nitrifying batch and continuous cultures. Two saline nitrifying batch cultures and two saline nitrifying continuous cultures were developed with two different chloride increasing approaches respectively. In the batch cultures, they were a gradual increase and a stepwise increase of the initial chloride concentrations, ranging from 0 to 30,000 mg/L. In the continuous cultures, they were a gradual increase and a constant level of the influent chloride concentrations, ranging from 0 to 18,200 mg/L. In addition to these saline cultures, both fresh batch and continuous cultures were also cultivated under no salinity conditions. Moreover, the change of nitrifiers population in both the fresh and saline cultures was also studied using the fluorescent in-situ hybridization (FISH) technique with thirteen 16S rRNA-targeted oligonucleotide probes. Due to time and personnel constraints, the batch cultures were only selected for the population study. It has been found that when the chloride concentration exceeded 6,500 mg/L, the fresh nitrifying batch culture was affected, while in the saline nitrifying batch cultures, both the chloride increasing approaches did not affect the nitrifying activity if the chloride concentration was not greater than 10,000 mg/L. However, when the chloride concentration increased from 10,000 to 20,000 mg/L, the activity was declined in both the batch cultures. Comparatively, the stepwise increase approach reduced the nitrifying activity more significantly than the gradual increase approach. When the chloride concentration was further increased to 30,000 mg/L, the stepwise increase approach performed slightly better than the gradual increase approach in terms of the activity maintenance. The nitrifying activities in both the batch cultures could be maintained at around 60% of the fresh culture even when the chloride concentration reached 30,000 mg/L. In the saline nitrifying continuous cultures, the gradual increase approach achieved a better ammonia removal efficiency than the constant level approach. It was confirmed that the chloride concentration of 18,200 mg/L in the influent did not affect the performance of both the continuous cultures compared to the control line without increasing influent salinity. However, the gradual increase approach needed a longer stabilization time (62 days). In both the saline batch and continuous cultures, it has been observed that when the chloride strength ranged from 10,000 to 18,000 mg/L, the nitrifying activity changed in a down-and-up pattern. This has been reasoned by the change of the dominant species of ammonia-oxidizers from non saline-resistant to saline-resistant species during a transitional period with an increase of chloride level, which has been demonstrated that Nitrosomonas europaea-lineage, Nitrosomonas eutropha, and Nitrosomonas halophila were the dominant species of ammonia-oxidizers in the nitrifying batch culture until the chloride concentration did not exceed 10,000 mg/L. After this level, they were faded out and replaced by Nitrosococcus mobilis-lineage. In the change of nitrite-oxidizers, Nitrobacter species were the only dominant ones when the chloride concentration was not greater than 10,000 mg/L. However, a further increase of chloride was found actually unable the nitrite-oxidizers survival. In a wastewater treatment plant, a better understanding of microbiology, ecology, and population dynamics of nitrifying bacteria is essential for improving process dynamics and control. Therefore, the change of nitrifying population with the increase of chloride concentration are particularly important in understanding the dynamic response of nitrifying microbial community to increasing chloride strength and also facilitating the development of an effective approach to build up robust saline-resistant nitrifiers in highly saline wastewater. In this regards, this research is meaningful. In a practical sense, the findings generated from this study has clearly endorsed that the saline sewage with a chloride strength less than 10,000 mg/L can be effectively treated through a nitrification process without having special approach to develop nitrifiers. This is specially beneficial to the Hong Kong sewage treatment since the chloride level of wastewater is usually not greater than 10,000 mg/L. Keywords: 16S rRNA-targeted oligonucleotide probes; chloride concentration; fluorescent in-situ hybridization (FISH); nitrifying activity; nitrifying batch culture; nitrifying continuous culture; saline wastewater.