  A criterion to define a pure pair-ion (PI) plasma is presented. It is
suggested that the lighter elements (like H and He) are more suitable to
produce PI plasmas. The observation of ion acoustic wave (IAW) in recent
experiments with fullerene plasmas clearly indicates the presence of electrons
in the system. A set of two coupled non-linear differential equations has been
obtained for PI plasma dynamics. In moving frame, it can be reduced to a form
similar to Hasegawa-Mima equation but it does not contain drift wave.
