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Ammonia (referring to both NH3 and NH4 (+)), is one of the most toxic nitrogen forms, and is toxic to many aquatic organisms. The present study identified a heterotrophic nitrifier Proteus mirabilis strain V7, isolated from the coastal seawater. Almost 100 % of 82 mg L-1 NH4 (+)-N was removed within 48 h, while 51.2 % of total inorganic nitrogen was removed within 96 h after inoculation of Proteus mirabilis strain V7. Moreover, Proteus mirabilis strain V7 could remove NH4 (+)-N in a seawater matrix, but with much lower efficiency. Three water samples containing NH4 (+)-N at concentrations of 64.40, 18.00 and 9.82 mg L-1 were chosen to further test the NH4 (+)-N removal ability of Proteus mirabilis strain V7. All of the NH4 (+)-N was removed, with only a trace production of NO2 (-) and NO3 (-). A putative subunit of ammonia monooxygenase from Proteus mirabilis strain V7, amoA (Pm), was cloned and its mRNA level in the presence of NH4 (+) was characterized by real time reverse transcriptase PCR (RT-PCR). A 1.8-fold increase in the mRNA level of amoA (Pm) was observed, which indicated that AmoA may be involved in NH4 (+) oxidization. Our results indicated that Proteus mirabilis strain V7 could potentially be used as new resource for the treatment of NH4 (+)-N contaminated environments.