Structure and morphology of lamellar crystals of two polyamides derived from 2-methylpentamethylenediamine (MPMD) and dibasic acids with 4 or 8 methylenes have been studied by transmission electron microscopy, electron diffraction and X-ray diffraction. The crystallographic unit cell of MPMD-6 is triclinic with a = 4.85, b = 5.90, c (chain axis) = 30.4 Angstrom, alpha = 115.40, beta = 99.9 and gamma = 61.1 degrees, the chain axis direction is not coincident with the c* reciprocal axis. These dimensions suggest a hydrogen-bonded sheet structure with a single hydrogen bond direction and a twist of 180 degrees in the odd diamine unit. MPMD-10 shows two crystalline structures. The most abundant being a monoclinic unit cell with a = 4.79, b = 5.67, c (chain axis) = 36.3 Angstrom and gamma = 61.8 degrees. The less abundant structure was only observed in oriented fibers and was similar to the MPMD-6 crystalline structure. Single crystals of both MPMD-6 and MPMD-10 polyamides grow preferentially along their single hydrogen bond direction.