Bacterial biofilters used in marine recirculation aquaculture systems need improvements to enhance nitrogen removal efficiency. Relatively little is known about biofilter autochthonous population structure and function. The present study was aimed at isolating and characterizing an autochthonous denitrifying bacterium from a marine biofilter installed at a recirculation aquaculture system. Colonization of four different media in a marine fish farm was followed by isolation of various denitrifying strains and molecular classification of the most promising one, strain T2, as a novel member of the Pseudomonas fluorescens cluster. This strain exhibits high metabolic versatility regarding N and C source utilization and environmental conditions for growth. It removed nitrate through aerobic assimilatory metabolism at a specific rate of 116.2 mg NO 3-N g dw-1 h-1. Dissimilatory NO3-N removal was observed under oxic conditions at a limited rate, where transient NO2-N formed represented 22% (0.17 mg L-1) of the maximum transient NO2-N observed under anoxic conditions. Dissimilatory NO3-N removal under anoxic conditions occurred at a specific rate of 53.5 mg NO3-N g dw-1 h-1. The isolated denitrifying strain was able to colonize different materials, such as granular activated carbon (GAC), Filtralite and Bioflow plastic rings, which allow the development of a prototype bioreactor for strain characterization under dynamic conditions and mimicking fish-farm operating conditions.