An apparatus for the isokinetic sampling of the liquid and gaseous phases in a fluid multiphase stream, comprising: - a sampling device for the separation of said stream in a sampled fraction having a flow-rate q C and a non-sampled fraction having a flow-rate q NC and for the sampling under substantially isokinetic conditions of a portion of fluid of said sampled fraction, said device comprising a tubular body , inside of which said multiphase stream is to flow, and a sampling probe having an upper end inside said tubular body open to the flow of said multiphase stream, and extending outside of said tubular body through an opening on it, - two differential pressure measuring means in fluid communication with the interior of said tubular body for simultaneously measuring the pressure drops Δp C and Δp NC of said sampled and non-sampled fractions, caused by flow restrictions; - means for the separation of the liquid and gaseous phases of said portion of sampled fraction; - means for the measurement of the flow-rates of the liquid and gaseous phases exiting from said separation means; said device being further provided with two concentric flow restrictions arranged parallel to each other, a first restriction positioned at said upper open end of said sampling probe and a second restriction positioned at the same height when the apparatus is in use and the tubular body is placed in a vertical position as said upper open end inside said tubular body, said restrictions having substantially an annular form ( or ), and being such as to cause said pressure drops and to define two narrowed, concentric flow sections of circular perimeter that constitute respectively a sampling opening having a narrowed section of area A C for the flowing of said sampled fraction and a non-sampling opening having a narrowed section of area A NC for the flowing of said non-sampled fraction, said sections being such that Δ p c = α C ⋅ q C 2 / A C 2 and Δ p NC = α NC ⋅ q NC 2 / A NC 2 wherein α C and α NC are a calibration coefficient of the section restriction through which the sampled fraction and, respectively, the non-sampled fraction flow, and said sections being such that Δ p c = φ Δ p NC and α C = φα NC , wherein the coefficient φ is constant. - a sampling device for the separation of said stream in a sampled fraction having a flow-rate q C and a non-sampled fraction having a flow-rate q NC and for the sampling under substantially isokinetic conditions of a portion of fluid of said sampled fraction, said device comprising a tubular body , inside of which said multiphase stream is to flow, and a sampling probe having an upper end inside said tubular body open to the flow of said multiphase stream, and extending outside of said tubular body through an opening on it, - two differential pressure measuring means in fluid communication with the interior of said tubular body for simultaneously measuring the pressure drops Δp C and Δp NC of said sampled and non-sampled fractions, caused by flow restrictions; - means for the separation of the liquid and gaseous phases of said portion of sampled fraction; - means for the measurement of the flow-rates of the liquid and gaseous phases exiting from said separation means; <EOS>