Abstract : Superheavy elements have been searched for by neutron induced fission of mass separated samples. Various natural materials have been investigated : minerals, manganese nodules, lunar dust, meteoritic materials. Fissioning masses have been collected in the A = 300 mass region. However, one cannot conclude that these masses are superheavy elements because of a possible contamination of this mass region by molecules containing natural uranium or thorium. To test for this possibility, the ratios of fast to thermal neutron events have been determined in each separated mass region. These ratios cannot be completely understood as due to the fission of U or Th atoms. Some complementary results concerning the properties of the atoms contained in these masses are also given.