A unitary rear frame mounting an engine and hydrostatic transmissions to the rear of a ZTR mower is disclosed. The unitary rear frame includes a lower mounting surface supporting the engine, an upper mounting surface above and forward of the lower mounting surface; a wall extending between the upper and lower mounting surfaces; and a pair of integral side panels depending downwardly from the upper mounting surface and extending forwardly of the wall. Each side panel may have a side pocket for mounting a hydrostatic transmission thereto.