Isotopic abundances of the elements xenon, cesium, barium, cerium, neodymium, samarium, rubidium and strontium formed in the fast (fission spectrum) neutron fission of u^238 have been measured using the mass spectrometric method. These ratios were normalized 'With respect to each other through isobaric nuclides and isotope dilution to obtain the relative yields of isobaric chains in the heavy mass region. By normalizing the heavy mass yields to 100%, the absolute fission yields of twenty-one isobaric Chains in the 130 - 154 mass range were determined. The origin of the fine structure in the cumulative yield versus mass curve is discussed. It has been concluded that, while most of the fine structure arises from the variation in neutron emission probabilities, some of it could be the result of the shell effect in the fission process itself. / Thesis / Doctor of Philosophy (PhD)