We report the unambiguous optical spectroscopic observation of spontaneous long-range crystal ordering in AlGaInP epitaxial layers grown by metalorganic vapor phase epitaxy. Polarized spectroscopy demonstrates the existence of an in-plane anisotropy for samples grown on substrates oriented close to (100). This anisotropy results from the order-induced splitting of the normally degenerate zone-center valence-band states. Our measurements allow the size of the valence-band splitting to be measured and also allow a determination of the order-induced band-gap reduction. Both of these quantities are found to decrease with increasing misorientation of the substrate away from (100), indicating that the degree of ordering also decreases with increasing misorientation.