The crystal structure of an acidic phospholipase A 2 from Ophiophagus hannah (king cobra) has been determined by molecular replacement at 2.6-Å resolution to a crystallographic R factor of 20.5% (R f r e e =23.3%) with reasonable stereochemistry. The venom enzyme contains an unusual ''pancreatic loop.'' The conformation of the loop is well defined and different from those in pancreas PLA 2 , showing its structural variability. This analysis provides the first structure of a PLA 2 -type cardiotoxin. The sites related to the cardiotoxic and myotoxic activities are explored and the oligomer observed in the crystalline state is described.