Phosphotriesterase (PTE) receives attention because it seems to be associated with the detoxication of organophosphorous pesticides and organophosphate resistance mechanism. In order to understand the biodegradation of phosphotriester pesticides and its signicance in the earthworm, a major non-target animal of pesticides, selected properties of phosphotiriesterase activity derived form the crude extract of Eisenia andrei were investigated. Pte activity appeared to be primarily localized in intestinal tissues. The highest level of PTE activity was found in epithelial tissue. THE NATIVE MOLECULAR WEIGHT OF EARTHWORM pte WAS 260 kDa and the isoelectric point was approximately 4. The optimal PH was approximately 9. The earthworm PTE had a substrate anity fof paraoxon with im value in the millimolar range. The presence of EGTA and EDTA completely abolished the activity and replacement of ca2+ ion is esential to maintain the activity.