High-resolution mass spectrometry indicates that the isotopic abundance of 50V of the Late Cretaceous La Luna petroleum asphaltenes of marine origin (highly enriched with V > 2000 ppm) is higher by about 3.5% than that of the inorganic source (VOSO4 x 5H2O, Merck). We propose that the difference in the 50V/51V values between the La Luna petroleum asphaltenes and the inorganic source can be best ascribed to the biological processing of seawater vanadium. The fact that the V isotopic compositions of petroleum asphaltenes vary over a very narrow range (2.46-2.52) suggests essentially the same (or similar) and fixed biological source of vanadium.