THE primordial abundances of the light elements and their isotopes provide essential information regarding the nucleosynthetic processes that occurred in the Big Bang(1,2), At present the best estimates of the baryon/photon ratio of the Universe, a fundamental cosmological parameter, are extrapolations to primordial times of light-element abundances measured in Solar System objects and in the Milky Way. Here we report a direct measurement of the He-3/He-4 abundance ratio in the local interstellar cloud, based on the isotopic analysis of helium atoms that have entered the Solar System from the surrounding interstellar medium. Our observations, together with the local-cloud D/H ratio(3-5) and Solar System light-element data(2), permit an improved estimate of the baryon/photon ratio, We find that, within present limits of error, the sum of the He-3 and deuterium abundances has remained essentially unchanged since the formation of the Solar System, placing new constraints on models of Galactic chemical evolution(6).