The 'external' oxygenated functional unit RtH2-e of the Rapana hemocyanin subunit RHSS2 was isolated and crystallized. X-ray intensity data to 3.3 A resolution have been collected at 100 K and the structure has been solved using the molecular-replacement method. The space group is assigned to be the tetragonal P4(3)2(1)2, with unit-cell parameters a = b = 105.5, c = 375.0 A.