The structural phase transitions occurring in PbZrO3 have been investigated using X-ray diffraction and dielectric constant measurements. The lattice parameters, Pb ion displacements and ZrO6 octahedral tilts in the antiferroelectric, orthorhombic phase have been determined as functions of temperature between 20 and 228 degrees C using continuously recording X-ray diffraction photography. The temperature dependence of the Pb ion displacement is discussed with reference to the Kittel theory of antiferroelectricity. The lattice parameters and structural data derived from these results are used to describe the temperature dependence of the birefringence in PbZrO3.