The hemocyanin of the tiger shrimp,Penaeus monodon, was investigated with respect to stability and oxygen binding. While hexamers occur as a major component, dodecamers and traces of higher aggregates are also found. Both the hexamers and dodecamers were found to be extremely sta-ble against dissociation at high pH, independently of the presence of cal-cium ions, in contrast to the known crustacean hemocyanins.