An oligomeric arginase with a molecular weight of 205000 is present in the intestinal tissues of the polychaete annelid Pista pacifica. The presence of an active subunit with a molecular weight of 34000 was demonstrated. The enzyme specificity, effect of thiol reagents on activity, kinetic properties of the intact enzyme and the active subunit were investigated. Treatment of the arginase with EDTA results in its dissociation into an inactive subunit; the active oligomeric structure can be regenerated by addition of Mn(2+). The correlation of characteristics of arginase from a certain species and that species, mode of nitrotelism is discussed.