Either spontaneous or artificial ordering of semiconductor alloys into CuAu‐like, chalcopyrite, or CuPt‐like structures is predicted to be accompanied by a reduction in the direct band gaps relative to the average over the binaries. In this letter calculated results are presented for seven III‐V and II‐VI alloys. We identify the mechanism for this band‐gap narrowing as band folding followed by repulsion between the folded states. The latter is coupled by the non‐zinc‐blende component of the superlattice potential. The same physical mechanism (but to a different extent) is responsible for gap bowing indisorderedalloys.