A series of crystal structures of the ethers, esters and carbonates of cholesterol were examined to obtain the structural information relevant to the liquid crystalline phases and the possible modes of association between the cholesterol derivatives and themselves as well as with other substances in biological systems. An examination of the unit cell parameters of the cholesterol derivatives suggests that the majority of the derivatives might have one of three common crystal packing arrangements, so called monolayer type I, type II and bilayer. The crystal data of cholesteryl ethylether and isopropylether obtained in this study indicate that those ethers belong to monolayer type II. Therefore, the primary aim of this study is to obtain the structural information on the conformation and the mode of the molecular interactions. The bond lengths and angles of cholesteryl ethylether and isopropylether are in good agreement, within experimental error, with those found in other cholesterol derivatives (see Table 1). The bond lengths in the C17 tails and ether chains show the apparent shortening which is characteristic of cholesterol derivatives, and is caused by the high thermal vibrations in these regions. In this case, it is especially pronounced in the C25-C27, C28-C29 and C28-C30 bond distances. The values of the atomic thermal parameters are unusually large, particularly in the ether groups and the terminal isopropyl group, C25, C26 and C27. The atoms, C3, O3, C28 and C29 lie in the same plane, with the torsion angle about O3-C28 being 176.0 (5)° for ethylether and 175.6(9)° for isopropylether, respectively. The isoprenoid chain at C17 is almost fully extended. The seven atoms of C17, C20, C22-C26 are in a zigzag planar chain and C21 and C27 are out of the plane. The crystal structures of cholesteryl ethylether and cholesteryl isopropyl ether (Fig. 2) consists of antiparallel molecules arranged to form monolayers which are parallel to the crystal (001) planes. Such monolayers are called type II. The monolayers are regions of closely packed semi-rigid cholesteryl ring systems that are separated by interface regions where the ether group atoms are more loosely packed. This packing type is similar to those of cholesteryl hexanoate, heptanoate, octanoate, pentyl carbonate, hexyl carbonate, oleate and chloroformate. The difference in the monolayer thickness (11.10-18.76 A) is associated primarily with structural differences in the interface region. The atoms in this region consist of the ester, carbonate or ether attached to O3 of cholesterol molecules. Most of the layered structures show liquid crystalline states, but these were not observed in the crystals of cholesteryl isopropylether. Crystals of cholesteryl isopropylether melt to give an isotropic liquid at a high temperature, 131.4 C. On cooling the crystals, freezing occurs at a temperature (117.1 C) which is considerably lower than the melting point. In cholesteryl ethylether, melting occurs at 87.7 C. The cholesteric phase forms only on cooling (73.3 C). The freezing point of the cholesteryl ethylether is