  It is generally believed that isospin would diminish in its importance as we
go towards heavy mass region due to isospin mixing caused by the growing
Coulomb forces. However, it was realized quite early that isospin could become
an important and useful quantum number for all nuclei including heavy nuclei
due to neutron richness of the systems~\cite{robson}. Lane and
Soper~\cite{lane} also showed in a theoretical calculation that isospin indeed
remains quite good in heavy mass neutron rich systems. In this paper, we
present isospin based calculations~\cite{jain,swati} for the fission fragment
distributions obtained from heavy-ion fusion fission reactions. We discuss in
detail the procedure adopted to assign the isospin values and the role of
neutron multiplicity data in obtaining the total fission fragment
distributions. We show that the observed fragment distributions can be
reproduced rather reasonably well by the calculations based on the idea of
conservation of isospin. This is probably the very first direct experimental
evidence of the validity of isospin in heavy nuclei, which arises largely due
to the neutron-rich nature of heavy nuclei and their fragments. This result may
eventually become useful for the theories of nuclear fission and also in other
practical applications.
