NOVELTY - Biodegradation of a dissolved hydrocarbon comprises introducing an aqueous stream containing at least one dissolved hydrocarbon and a dissolved oxide of nitrogen to a flow-through reactor, exposing a denitrifying culture capable of degrading at least one dissolved hydrocarbon to a dissolved hydrocarbon in the reactor which is operated at 10-65 degrees C with mean cell residence time of at least 15 hours.    USE - For treating water contaminated with dissolved hydrocarbons e.g. metabolizing saturated, unsaturated, aromatic and/or halogenated hydrocarbons from wastewater, groundwater, sludge or slurry. The invention can be used in situ or water can be pumped out of the ground, treated and then pumped back underground (pump-and-treat).    ADVANTAGE - The process is efficient and cost-effective.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) apparatus for carrying out the process of biodegradation of a dissolved hydrocarbon; and    (2) treating an aqueous stream comprising:    (a) a first treatment step of introducing the aqueous stream comprising a dissolved aromatic hydrocarbon and a dissolved chlorinated hydrocarbon to a continuous-stirred tank reactor at a rate;    (b) a second treatment step of exposing the aqueous stream in the reactor to a microorganism culture capable of growing on toluene or benzene as a sole source of carbon; and    (c) a third treatment step of removing the aqueous stream from the reactor at a rate selected so the dissolved aromatic hydrocarbon is reduced to less than 0.2 mg/l and dissolved chlorinated hydrocarbon reduced to less than 0.01 mg/l.    DESCRIPTION OF DRAWING(S) - The figure shows a preferred apparatus for biodegradation of a dissolved hydrocarbon.    apparatus (100)    contaminated underground water (102)    underground formation (104)    extraction well (106)    pump (108)    dissolved nitrate (110)    above-ground reactor (114)    sedimentation tank (116)    injection well (120)    pump (124)