Endolysin from the phage Cp-1 (Cpl-1) cleaves the glycosidic beta1,4-bonds between the N-acetylmuramic acid and the N-acetylglucosamine of the pneumococcal cell wall. Cpl-1 has been crystallized using the hanging-drop vapour-diffusion method at 291 K. Diffraction-quality orthorhombic crystals of the native protein were obtained only after addition of the detergent n-decyl-beta-D-maltoside. Crystals belong to space group C222(1), with unit-cell parameters a = 77.949, b = 95.782, c = 129.282 A. Diffraction data to a resolution of 2.1 A were collected at a synchrotron facility.