The thermopower and resistivity of a fully loaded and a partially unloaded crystal of YBa2Cu3O7- (Y123) have been measured along the a and b axes. The temperature dependence of the a-axis thermopower in the fully loaded crystal is similar to that observed previously in crystalline and polycrystalline Y123, and rather smaller than is found in other high-Tc cuprates. The thermopower along the b axis, which is affected more strongly by the chains, shows substantial variation between the present and previous data, and we ascribe this to chain disorder. In the unloaded crystal there is a nearly temperature-independent shift to larger thermopower along the a axis, while the b-axis thermopower shows in addition an altered temperature dependence. © 1995 The American Physical Society.