Dielectric, X-ray diffraction, and domain structure evolution for Mn-doped sodium bismuth titanate (Na0.5Bi0.5TiO3) single crystals was investigated as a function of temperature. A notable thermal hysteresis was found at the tetragonal -> rhombohedral transition between heating and cooling measurements for the structural and dielectric properties. Domain studies by polarized light microscopy (PLM) revealed a corresponding thermal hysteresis at an isotropization point, where the contrast in the images disappeared but reappeared both at higher and lower temperatures. The images below the isotropization point exhibited stripe-like color bands, whose contrast became more intense and whose formation temperature on cooling was increased under electric field. Studies by piezoelectric force microscopy revealed fine-scale ferroelectric domains of about 0.1-0.3 mu m in size that existed within these color bands at room temperature.