The structure and morphology of nylon 68 single crystals were studied by transmission electron microscopy. There are two crystal phases. A monoclinic phase with a = 0.960 +/- 0.005 nm, b = 0.826 +/- 0.005 nm, and gamma = 115 +/- 1 degrees, when viewed along the chain axis, c, is the usual form for crystals cooled slowly from the crystallization temperature. This lattice is the same, within experimental error, as that of monoclinic crystals of nylon 46, a polymer which has the same chain structure, but with amide groups more closely spaced. Nylon 68 crystals quenched into nonsolvent from the crystallization temperature are found in a pseudohexagonal phase which has parameters a = b = 0.97 nm and gamma = 120 degrees. When monoclinic crystals are heated, they transform, gradually, into the pseudohexagonal phase; the transformation is complete at 203 degrees C, and melting takes place at 234 degrees C. The crystals grow from solution in the pseudohexagonal phase which is stable at high temperatures; they usually revert to the monoclinic phase on cooling, but the pseudohexagonal phase can be obtained at room temperature by quenching to below the glass transition temperature. Monoclinic nylon 46 single crystals were examined on heating. They transformed to the pseudohexagonal structure at 245 degrees C, before melting took place at 295 degrees C.