  We show that the isotropic lines in the lattice Z_{d}^{2} are the Lagrangian
submodules of that lattice and we give their number together with the number of
them through a given point of the lattice. The set of isotropic lines decompose
into orbits under the action of SL(2,Z_d). We give an explicit description of
those orbits as well as their number and their respective cardinalities. We
also develop two group actions on the group \Sigma_{D}(M) related to the topic.
