A fixture is described for collecting X‐ray diffraction data from biomolecule crystals under gas pressures up to 300 atm (30 MPa). The fixture is completely portable and can be used on conventional diffractometers. Normal procedures for crystal mounting, alignment and data collection require only slight modification. A test data set of the protein sperm whale metmyoglobin under a gas pressure of 145 atm of N2gas has been collected to a nominal resolution of 2.0 Å using CuKαradiation and refined to anR‐factor of 16.2%. Data collection beyond 2.0 Å resolution is hampered by the presence of strong powder diffraction from the