An x‐ray diffractometer has been constructed so as to allow the study of cyrogenic fluids, in particular liquid helium, at temperatures below 4.2 K. The design and operating characteristics of the diffractometer are presented. Data acquisition is automatic. The x‐ray beam is almost exclusively in vacuum and the slits which define the beam are adjustable while the vacuum integrity is maintained. Monochromatization of the x‐ray beam is accomplished without use of crystal diffraction.