Of the light elements, the primordial abundance of deuterium relative to hydrogen, (D/H)(p), provides the most sensitive diagnostic(1) for the cosmological mass density parameter, Omega(B). Recent high-redshift D/H measurements are highly discrepant(2-6), although this may reflect observational uncertainties(7,8). The larger primordial D/H values imply a low Omega(B) (requiring the Universe to be. dominated by non-baryonic matter), and cause problems for galactic chemical evolution models, which have difficulty in reproducing the steep decline in D/H to the present-day values, Conversely, the lower D/H values measured at high redshift imply an Omega(B) greater than that derived from Li-7 and He-4 abundance measurements, and may require a deuterium-abundance evolution that is too low to easily explain. Here we report the first measurement of D/H at intermediate redshift (z = 0.7010), in a gas cloud selected to minimize observational uncertainties, Our analysis yields a value of D/H ((2.0 +/- 0.5) x 10(-4)) which is at the upper end of the range of values measured at high redshifts. This finding, together with other independent observations, suggests that there may be inhomogeneity in (D/H)(p) of at least a factor of ten.