Chemical ordering in cubic epitaxial InxGa1-xN layers is investigated by combining first-principles pseudopotential plane-wave total-energy calculations, a local concentration-dependent cluster-based method, and Monte Carlo simulations. It is found that for the unstrained or fully relaxed layers there are no stable ordered structures, indicating the tendency of the alloy to undergo phase separation, in agreement with previous calculations and experiment. The energetics of the InxGa1-xN layers pseudomorphycally grown on fully relaxed GaN (001) buffers shows that biaxial strain acts as the driving force for chemical ordering in the alloys. It is found that strained InxGa1-xN alloy comprises stable ordered structures which are (210)-oriented superlattices with composition in the range [0.5,0.63], the [AABB] alternation of planes (configuration "chalcopyrite") being the most stable phase.6924