A method for determining a first flow rate of a gas phase and a second flow rate of at least one liquid phase present in a polyphasic fluid circulating in a conduit is presented. The method essentially comprises circulating the polyphasic fluid through the neck of a venturi delimited by the conduit. The method further comprises estimating the first flow rate and the second flow rate by using a measured pressure difference of the fluid through the neck and a property representative of the relative surface occupied by the gas phase relative to a total surface over a cross-section of the neck. The step of estimating the first flow rate and the second flow rate further comprises (a1) calculating an amount representative of the liquid phase present in the gas rich core relative to the total amount of liquid phase in the neck and calculating the first and second flow rate as a function of the amount representative of the liquid phase present in the core calculated during step (a1).