We present results of optical investigations of the {100} and {101} faces of KDP. A technique based on the optical Schlieren method is developed to in situ investigate the growing crystal morphology under real conditions of rapid profiled growth. It is shown that the structure of growth centers on the growing surface essentially depends on the growth conditions, which provides additional possibilities of the crystal quality control. Some experimental results on step bunches are interpreted in terms of the "density shock wave" model. (C) 2003 Elsevier B.V. All rights reserved.