An apparatus for the isokinetic sampling of the liquid and gaseous phases in a fluid multiphase stream, comprising: - a sampling device (3, 20) 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 (13,21), inside of which said multiphase stream is to flow, and a sampling probe (15,24) 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 (7C,7NC) 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 (17,26) positioned at said upper open end of said sampling probe and a second restriction (14,23) 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 ((14,17) or (23,26)), 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 (18,27) having a narrowed section of area A C for the flowing of said sampled fraction and a non-sampling opening (16,25) 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 (3, 20) 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 (13,21), inside of which said multiphase stream is to flow, and a sampling probe (15,24) 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 (7C,7NC) 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;