We have carried out first-principles pseudopotential calculations to study the formation energies of various ZnSe(100) surface reconstructions. Both Zn- and Se-terminated models are treated, using as a guide the satisfaction of the electron counting rule (i.e., charge compensation at the surface by the filling of all the Se dangling bonds and the emptying of the Zn dangling bonds). In order to treat structures with different stoichiometries, we determine the surface energy as a function of the Zn chemical potential, which is allowed to vary in a range determined by the energies of bulk Zn, Se, and ZnSe. A c(2 X 2) reconstruction with half-monolayer coverage of twofold coordinated Zn atoms is stable in the Zn-rich limit. Under moderately Se-rich conditions, the surface adopts a (2 X 1) Se-dimer phase. In the extreme Se-rich limit we predict the stability of a reconstruction with one and a half monolayer coverage of Se. The new structure can account for the high growth rates observed in atomic layer epitaxy and migration enhanced epitaxy experiments at relatively low temperatures.