Using the Landau-Ginzburg-Devonshire approach, the longitudinal piezoelectric coefficient in an arbitrary direction, d(33)(*)(theta), was calculated as a function of temperature in tetragonal BaTiO3 crystals. The direction along which d(33)(*)(theta) is maximum is a function of piezoelectric d(ij) coefficients referred to the crystallographic coordinate system. Below a critical ratio of the shear and longitudinal coefficients, d(15)/d(33), the maximum d(33)(*)(theta) lies along the  axis. As the low-temperature orthorhombic phase is approached on cooling, the d(15) increases, reflecting softening of the lattice along the axis of the incipient orthorhombic distortion, and the direction of maximum d(33)(*)(theta) deviates significantly from the  axis. Our results suggest that the enhanced d(33)(*)(theta) coefficient along a direction other than the polar axis recently reported in some ferroelectrics is at least in part controlled by these intrinsic lattice effects. (C) 2002 American Institute of Physics.