NOVELTY - The process for catalytic hydrotreatment of gas oil feedstock and biological origin of vegetable oil- and animal fat feedstock (less than 15 wt.%) in a unit with fixed bed, comprises injecting the oil feedstock in a first reactor, injecting a part of the biological feedstock in the first reactor and other part in a second reactor, injecting a liquid effluent withdrawn from the first reactor in the second reactor, and effecting a supplementary treatment of carbon monoxide present in the gas recycled from the hydrotreatment of the total feedstock.    USE - The process is useful for catalytic hydrotreatment of gas oil feedstock and biological origin of vegetable oil- and animal fat feedstock in a unit with fixed bed (claimed).    ADVANTAGE - The process is capable of effectively and economically hydrotreating the gas oil feedstock and biological origin of vegetable oil- and animal fat feedstock, minimizes the formation of methane, and increases the properties such as cetane number, density, volume and distillation of oil product.    DETAILED DESCRIPTION - The process for catalytic hydrotreatment of gas oil feedstock and biological origin of vegetable oil- and animal fat feedstock (less than 15 wt.%) in a unit with fixed bed, comprises injecting the oil feedstock in a first reactor, injecting a part of the biological feedstock in the first reactor and other part in a second reactor, injecting a liquid effluent withdrawn from the first reactor in the second reactor, effecting a supplementary treatment of carbon monoxide present in the gas recycled from the hydrotreatment of the total feedstock, and controlling the exothermicity of the biological feedstock hydrotreatment using a thermal regulation system. The gas oil feedstock is a gas fraction originating from distillation of crude oil and/or synthetic crude obtained from the treatment of bituminous schists, other heavy crude oils or effluent of Fischer Tropsch process, and originating from catalytic and/or thermal cracking (fluid catalytic cracking, coking, visbreaking). The catalytic injection zone of the biological feedstock comprises a first catalytic bed trap with metals. A quantity of hydrogen (120-450 Normoliters per liters of gas oil feedstock, and 200-1000 Normoliters per liters of biological feedstock) is introduced in the first and second catalytic zone respectively. The gas oil feedstock and biological feedstock are treated in the first and second catalytic zone at a temperature of 340-400 degrees C and 280-350 degrees C and at a pressure of 30-70 bars respectively. Spatial speed of the oil feedstock and biological feedstock in the first and second zone is 0.6-3 and 1-5 respectively. The biological feedstock crosses a catalytic bed having a catalyst in the hydrotreatment unit. Water is injected at a level of the biological feedstock treatment zone. The carbon monoxide is treated by a conversion unit using a reaction of carbon monoxide shift. A pressure swing adsorption unit is arranged for separating methane, ethane and propane, and the separated compounds are used in steam methane reformer. Carbon dioxide and hydrogen sulfide present in the recylce gas is treated or separated before re-injecting the gas in the hydrotreatment unit. The thermal regulation system has a liquid at a level of the biological feedstock treatment zone, and recuperates the heat of the effluent in the first reactor thus lowering the effluent temperature before injecting it to the second reactor. The hydrotreatment unit operates in a manner such that no liquid effluent recycles at the top of the reactor. INDEPENDENT CLAIMS are included for:    (1) a catalytic hydrotreatment unit; and    (2) a hydroforming unit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the hydrotreatment unit.    Reactor (20)    First inlet (21)    Second inlet (22)    First hydrogen inlet (23)    Second hydrogen inlet (24)    Water inlet. (25)