The presence of C nanoparticles in the asphaltenes precipitated from a crude oil from the sureste Basin in Mexico is reported. Most of the near spherical nanoparticles were identified as the 3C cubic polytype of carbon (n-diamond). A second type was found in much smaller quantities and identified as the 2H hexagonal polytype of diamond. The direct conversion of petroleum into nanodiamonds was ruled out on the basis of the high temperature (greater than or equal to 1400 degreesC) and pressures (greater than or equal to 5 GPa) required for the transformation. The nanodiamonds found may have had their origin in processes such as (a) the meteoritic impact shock waves acting on carbonaceous materials, (b) the deposition of a C plasma from a fireball produced by a meteoritic impact, or (c) the irradiation of the source material and/or the asphaltenes of the crude oil by highly energetic particles resulting from the nuclear fission of U and Th. It was also found that the available data did not allow an unambiguous identification of the process that generated the nanodiamonds.