We report the discovery of indigenous 'live' oil in ∼1.4-Gyr-old rocks in the McArthur Basin of northern Australia. Previously reported occurrences of indigenous Precambrian oil are < 1 Gyr old1-5. Potential petroleum source rocks in the McArthur Basin range in age from 1.4 to 1.7 Gyr and were deposited in marine and lacustrine environments. In parts of the basin they have been buried sufficiently deeply to have generated hydrocarbons. They span the period corresponding to the appearance of eukaryotic organisms, and because of their low degree of thermal alteration, they provide a valuable resource for the study of primitive biota through their hydrocarbon biomarkers. The hydrocarbon composition of the oil is consistent with a derivation from organic matter of prokaryotic origin. These results show that exploration of previously ignored mid-Proterozoic sediments may lead to the discovery of new reserves of oil.