Rapana thomasiana hemocyanin is a representative of molluscan (gastropodan) dioxygen-transporting proteins. The cylindrical hemocyanin aggregates are composed of two structural subunits, RHSS1 and RHSS2. The 420 kDa subunit RHSS2 contains eight 50-55 kDa functional units. Each unit has a single dioxygen-binding dinuclear copper-containing active site. Molluscan hemocyanin functional units can be subdivided into "internal units", forming the so-called "arch" inside the hemocyanin cylinders, and "external" units, building the cylinder wall of the aggregates. The "external" oxygenated functional unit RtH2-e of the Rapana hemocyanin subunit RHSS2 was isolated and crystallized in two crystal forms. Type I crystals are small but X-ray suitable and show bipyramidal morphology. Preliminary data were collected to 3.3 Angstrom at 120 K using synchrotron radiation. The space group is assigned to be the tetragonal P4(1)2(1)2 or its enantiomer with unit cell dimensions a = b = 105.5 Angstrom and c = 375.0 Angstrom. Type II crystals grow in thin plates and diffract to about 3.0 A. However, they are always twinned and cannot be utilized for data collection and structure analysis.