NOVELTY - A hydrocarbon fraction is produced from a mixture of biological origin containing esters of fatty acids, and free fatty acids, by hydrotreating the mixture of biological origin in the presence of a catalytic composition comprising an amorphous support of acidic nature, and a metallic component containing metal(s) of the group VIII mixed with metals of group VIB. The amorphous support of acidic nature is an amorphous silica-alumina having silicon dioxide/aluminum oxide molar ratio of greater than or equal to 5; or a porous solid material comprising silicon, aluminum, phosphorus and oxygen.    USE - Method for producing a hydrocarbon fraction used as diesel fuel or as a component of diesel fuel, from a mixture of biological origin containing esters of fatty acids, and free fatty acids (claimed).    ADVANTAGE - The method uses catalytic compositions which do not only allow a high-quality product to be obtained but also have characteristics to be used in the presence of water without undergoing deactivation or to be easily regenerated during the hydrotreatment itself by means of a moderate temperature rise.    DETAILED DESCRIPTION - Producing a hydrocarbon fraction from a mixture of biological origin containing esters of fatty acids, and free fatty acids, comprises hydrotreating the mixture of biological origin in the presence of a catalytic composition comprising an amorphous support of acidic nature, and a metallic component containing metal(s) of the group VIII mixed with metals of group VIB. The amorphous support of acidic nature is (1) an amorphous silica-alumina having silicon dioxide (SiO2)/aluminum oxide (Al2O3) molar ratio of greater than or equal to 5; or (2) a porous solid material comprising silicon (Si), aluminum (Al), phosphorus (P) and oxygen bonded together to form an amorphous mixed oxide comprising a single phase with a Si/Al atomic ratio of 15-250, P/Al ratio of greater than or equal to 0.1, but lower than 5, preferably 0.3-3.5, a total pore volume of 0.5-2 ml/g, an average pore diameter of 3-40 nm and a specific surface area of 200-1000, preferably 300-900 m2/g. With the proviso that the catalytic composition, if formed by a silica-alumina of (1) type and Pt, is pretreated with a hydrocarbon containing 7-16C, when the silica-alumina is a completely amorphous, micro-mesoporous silica-alumina (1a) having SiO2/Al2O3 molar ratio of 30-500, a surface area greater than 500 m2/g, a pore volume of 0.3-1.3 ml/g, and an average pore diameter smaller than 40 Angstrom . An INDEPENDENT CLAIM is included for acidic material in extruded form, containing an inorganic inert binder, an amorphous silica-alumina and phosphorus, which is obtained by:    (A) forming an aqueous solution of tetralkylammonium hydroxide (TAA-OH), a soluble aluminum compound capable of hydrolyzing in Al2O3 and a silicon compound of hydrolyzing in SiO2, where the molar ratios of SiO2/Al2O3 is 30/1 to 500/1, TAA-OH/SiO2 is 0.05/1 to 0.2/1, water (H2O)/SiO2 is 5/1 to 40/1;    (B) heating the obtained solution to cause hydrolysis and gelification and obtain a mixture having a viscosity of 0.01-100 Pa-second;    (C) bringing the mixture into contact with a binder from the bohemite or pseudobohemite class, in a weight ratio with the mixture comprising 0.05-0.5, and then with a mineral or organic acid at 0.5-8.0 g per 100 g of binder;    (D) heating the mixture under mixing, up to 40-90 degrees C until a homogenous paste is obtained that undergoes extrusion and granulation; and    (E) drying the extruded product, and calcining under oxidizing atmosphere.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a plant for producing hydrocarbon fractions.