Dislocations in monoclinic hen egg-white lysozyme crystals were investigated by means of synchrotron monochromatic-beam X-ray topography. The loop and curved dislocations were observed to be predominant in the crystals. Almost all the dislocations lay in (1 0 (1) over bar) crystallographic plane, which corresponds to that with smallest slicing energy estimated by macrobond approach. One of the Burgers vectors of the dislocations was determined to be [0 1 0], which corresponds to the smallest lattice translational vector on the (1 0 (1) over bar) plane. It is suggested that the loop and curved dislocations are slip ones introduced by a stress concentration during or after the growth. (C) 2010 Elsevier B.V. All rights reserved.